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[PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush

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  [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-05-19 16:20 +0200
    Re: [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush Andy Lutomirski <luto@kernel.org> - 2017-05-21 05:30 +0200
      RE: [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush KY Srinivasan <kys@microsoft.com> - 2017-05-22 16:40 +0200
      Re: [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush Andy Lutomirski <luto@kernel.org> - 2017-05-22 20:30 +0200
        Re: [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-05-23 14:40 +0200
          RE: [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush KY Srinivasan <kys@microsoft.com> - 2017-05-23 20:00 +0200

#1645633 — [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush

Hyper-V host can suggest us to use hypercall for doing remote TLB flush,
this is supposed to work faster than IPIs.

Implementation details: to do HvFlushVirtualAddress{Space,List} hypercalls
we need to put the input somewhere in memory and we don't really want to
have memory allocation on each call so we pre-allocate per cpu memory areas
on boot. These areas are of fixes size, limit them with an arbitrary number
of 16 (16 gvas are able to specify 16 * 4096 pages).

pv_ops patching is happening very early so we need to separate
hyperv_setup_mmu_ops() and hyper_alloc_mmu().

It is possible and easy to implement local TLB flushing too and there is
even a hint for that. However, I don't see a room for optimization on the
host side as both hypercall and native tlb flush will result in vmexit. The
hint is also not set on modern Hyper-V versions.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Acked-by: K. Y. Srinivasan <kys@microsoft.com>
Tested-by: Simon Xiao <sixiao@microsoft.com>
Tested-by: Srikanth Myakam <v-srm@microsoft.com>
---
 arch/x86/hyperv/Makefile           |   2 +-
 arch/x86/hyperv/hv_init.c          |   2 +
 arch/x86/hyperv/mmu.c              | 117 +++++++++++++++++++++++++++++++++++++
 arch/x86/include/asm/mshyperv.h    |   3 +
 arch/x86/include/uapi/asm/hyperv.h |   7 +++
 arch/x86/kernel/cpu/mshyperv.c     |   1 +
 6 files changed, 131 insertions(+), 1 deletion(-)
 create mode 100644 arch/x86/hyperv/mmu.c

diff --git a/arch/x86/hyperv/Makefile b/arch/x86/hyperv/Makefile
index 171ae09..367a820 100644
--- a/arch/x86/hyperv/Makefile
+++ b/arch/x86/hyperv/Makefile
@@ -1 +1 @@
-obj-y		:= hv_init.o
+obj-y		:= hv_init.o mmu.o
diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 7fd9cd3..df3252f 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -140,6 +140,8 @@ void hyperv_init(void)
 	hypercall_msr.guest_physical_address = vmalloc_to_pfn(hv_hypercall_pg);
 	wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
 
+	hyper_alloc_mmu();
+
 	/*
 	 * Register Hyper-V specific clocksource.
 	 */
diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
new file mode 100644
index 0000000..e3ab9b9
--- /dev/null
+++ b/arch/x86/hyperv/mmu.c
@@ -0,0 +1,117 @@
+#include <linux/types.h>
+#include <linux/hyperv.h>
+#include <linux/slab.h>
+#include <linux/log2.h>
+#include <asm/mshyperv.h>
+#include <asm/tlbflush.h>
+#include <asm/msr.h>
+#include <asm/fpu/api.h>
+
+/* HvFlushVirtualAddressSpace, HvFlushVirtualAddressList hypercalls */
+struct hv_flush_pcpu {
+	__u64 address_space;
+	__u64 flags;
+	__u64 processor_mask;
+	__u64 gva_list[];
+};
+
+static struct hv_flush_pcpu __percpu *pcpu_flush;
+
+static void hyperv_flush_tlb_others(const struct cpumask *cpus,
+				    struct mm_struct *mm, unsigned long start,
+				    unsigned long end)
+{
+	struct hv_flush_pcpu *flush;
+	unsigned long cur, flags;
+	u64 status = -1ULL;
+	int cpu, vcpu, gva_n, max_gvas;
+
+	if (!pcpu_flush || !hv_hypercall_pg)
+		goto do_native;
+
+	if (cpumask_empty(cpus))
+		return;
+
+	local_irq_save(flags);
+
+	flush = this_cpu_ptr(pcpu_flush);
+
+	if (mm) {
+		flush->address_space = virt_to_phys(mm->pgd);
+		flush->flags = 0;
+	} else {
+		flush->address_space = 0;
+		flush->flags = HV_FLUSH_ALL_VIRTUAL_ADDRESS_SPACES;
+	}
+
+	flush->processor_mask = 0;
+	if (cpumask_equal(cpus, cpu_present_mask)) {
+		flush->flags |= HV_FLUSH_ALL_PROCESSORS;
+	} else {
+		for_each_cpu(cpu, cpus) {
+			vcpu = hv_cpu_number_to_vp_number(cpu);
+			if (vcpu != -1 && vcpu < 64)
+				flush->processor_mask |= 1 << vcpu;
+			else
+				goto do_native;
+		}
+	}
+
+	/*
+	 * We can flush not more than max_gvas with one hypercall. Flush the
+	 * whole address space if we were asked to do more.
+	 */
+	max_gvas = (PAGE_SIZE - sizeof(*flush)) / 8;
+
+	if (end == TLB_FLUSH_ALL ||
+	    (end && ((end - start)/(PAGE_SIZE*PAGE_SIZE)) > max_gvas)) {
+		if (end == TLB_FLUSH_ALL)
+			flush->flags |= HV_FLUSH_NON_GLOBAL_MAPPINGS_ONLY;
+		status = hv_do_hypercall(HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE,
+					 flush, NULL);
+	} else {
+		cur = start;
+		gva_n = 0;
+		do {
+			flush->gva_list[gva_n] = cur & PAGE_MASK;
+			/*
+			 * Lower 12 bits encode the number of additional
+			 * pages to flush (in addition to the 'cur' page).
+			 */
+			if (end >= cur + PAGE_SIZE * PAGE_SIZE)
+				flush->gva_list[gva_n] |= ~PAGE_MASK;
+			else if (end > cur)
+				flush->gva_list[gva_n] |=
+					(end - cur - 1) >> PAGE_SHIFT;
+
+			cur += PAGE_SIZE * PAGE_SIZE;
+			++gva_n;
+
+		} while (cur < end);
+
+		status = hv_do_rep_hypercall(HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST,
+					     gva_n, 0, flush, NULL);
+
+	}
+
+	local_irq_restore(flags);
+
+	if (!(status & 0xffff))
+		return;
+do_native:
+	native_flush_tlb_others(cpus, mm, start, end);
+}
+
+void hyperv_setup_mmu_ops(void)
+{
+	if (ms_hyperv.hints & HV_X64_REMOTE_TLB_FLUSH_RECOMMENDED) {
+		pr_info("Hyper-V: Using hypercall for remote TLB flush\n");
+		pv_mmu_ops.flush_tlb_others = hyperv_flush_tlb_others;
+	}
+}
+
+void hyper_alloc_mmu(void)
+{
+	if (ms_hyperv.hints & HV_X64_REMOTE_TLB_FLUSH_RECOMMENDED)
+		pcpu_flush = __alloc_percpu(PAGE_SIZE, PAGE_SIZE);
+}
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index eb38da3..359967f 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -308,6 +308,8 @@ static inline int hv_cpu_number_to_vp_number(int cpu_number)
 }
 
 void hyperv_init(void);
+void hyperv_setup_mmu_ops(void);
+void hyper_alloc_mmu(void);
 void hyperv_report_panic(struct pt_regs *regs);
 bool hv_is_hypercall_page_setup(void);
 void hyperv_cleanup(void);
@@ -318,6 +320,7 @@ static inline bool hv_is_hypercall_page_setup(void)
 	return false;
 }
 static inline hyperv_cleanup(void) {}
+static inline void hyperv_setup_mmu_ops(void) {}
 #endif /* CONFIG_HYPERV */
 
 #ifdef CONFIG_HYPERV_TSCPAGE
diff --git a/arch/x86/include/uapi/asm/hyperv.h b/arch/x86/include/uapi/asm/hyperv.h
index c87e900..3d44036 100644
--- a/arch/x86/include/uapi/asm/hyperv.h
+++ b/arch/x86/include/uapi/asm/hyperv.h
@@ -239,6 +239,8 @@
 		(~((1ull << HV_X64_MSR_HYPERCALL_PAGE_ADDRESS_SHIFT) - 1))
 
 /* Declare the various hypercall operations. */
+#define HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE	0x0002
+#define HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST	0x0003
 #define HVCALL_NOTIFY_LONG_SPIN_WAIT		0x0008
 #define HVCALL_POST_MESSAGE			0x005c
 #define HVCALL_SIGNAL_EVENT			0x005d
@@ -256,6 +258,11 @@
 #define HV_PROCESSOR_POWER_STATE_C2		2
 #define HV_PROCESSOR_POWER_STATE_C3		3
 
+#define HV_FLUSH_ALL_PROCESSORS			0x00000001
+#define HV_FLUSH_ALL_VIRTUAL_ADDRESS_SPACES	0x00000002
+#define HV_FLUSH_NON_GLOBAL_MAPPINGS_ONLY	0x00000004
+#define HV_FLUSH_USE_EXTENDED_RANGE_FORMAT	0x00000008
+
 /* Hypercall interface */
 union hv_hypercall_input {
 	u64 as_uint64;
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index a8b4765..16a9221 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -240,6 +240,7 @@ static void __init ms_hyperv_init_platform(void)
 	 * Setup the hook to get control post apic initialization.
 	 */
 	x86_platform.apic_post_init = hyperv_init;
+	hyperv_setup_mmu_ops();
 #endif
 }
 
-- 
2.9.3

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#1646241

On 05/19/2017 07:09 AM, Vitaly Kuznetsov wrote:
> Hyper-V host can suggest us to use hypercall for doing remote TLB flush,
> this is supposed to work faster than IPIs.
> 
> Implementation details: to do HvFlushVirtualAddress{Space,List} hypercalls
> we need to put the input somewhere in memory and we don't really want to
> have memory allocation on each call so we pre-allocate per cpu memory areas
> on boot. These areas are of fixes size, limit them with an arbitrary number
> of 16 (16 gvas are able to specify 16 * 4096 pages).
> 
> pv_ops patching is happening very early so we need to separate
> hyperv_setup_mmu_ops() and hyper_alloc_mmu().
> 
> It is possible and easy to implement local TLB flushing too and there is
> even a hint for that. However, I don't see a room for optimization on the
> host side as both hypercall and native tlb flush will result in vmexit. The
> hint is also not set on modern Hyper-V versions.

Why do local flushes exit?

> +static void hyperv_flush_tlb_others(const struct cpumask *cpus,
> +				    struct mm_struct *mm, unsigned long start,
> +				    unsigned long end)
> +{

What tree will this go through?  I'm about to send a signature change 
for this function for tip:x86/mm.

Also, how would this interact with PCID?  I have PCID patches that I'm 
pretty happy with now, and I'm hoping to support PCID in 4.13.

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#1646985

> -----Original Message-----
> From: devel [mailto:driverdev-devel-bounces@linuxdriverproject.org] On
> Behalf Of Vitaly Kuznetsov
> Sent: Monday, May 22, 2017 3:44 AM
> To: Andy Lutomirski <luto@kernel.org>
> Cc: Stephen Hemminger <sthemmin@microsoft.com>; Jork Loeser
> <Jork.Loeser@microsoft.com>; Haiyang Zhang <haiyangz@microsoft.com>;
> x86@kernel.org; linux-kernel@vger.kernel.org; Steven Rostedt
> <rostedt@goodmis.org>; Ingo Molnar <mingo@redhat.com>; H. Peter Anvin
> <hpa@zytor.com>; devel@linuxdriverproject.org; Thomas Gleixner
> <tglx@linutronix.de>
> Subject: Re: [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB
> flush
> 
> Andy Lutomirski <luto@kernel.org> writes:
> 
> > On 05/19/2017 07:09 AM, Vitaly Kuznetsov wrote:
> >> Hyper-V host can suggest us to use hypercall for doing remote TLB flush,
> >> this is supposed to work faster than IPIs.
> >>
> >> Implementation details: to do HvFlushVirtualAddress{Space,List}
> hypercalls
> >> we need to put the input somewhere in memory and we don't really
> want to
> >> have memory allocation on each call so we pre-allocate per cpu memory
> areas
> >> on boot. These areas are of fixes size, limit them with an arbitrary number
> >> of 16 (16 gvas are able to specify 16 * 4096 pages).
> >>
> >> pv_ops patching is happening very early so we need to separate
> >> hyperv_setup_mmu_ops() and hyper_alloc_mmu().
> >>
> >> It is possible and easy to implement local TLB flushing too and there is
> >> even a hint for that. However, I don't see a room for optimization on the
> >> host side as both hypercall and native tlb flush will result in vmexit. The
> >> hint is also not set on modern Hyper-V versions.
> >
> > Why do local flushes exit?
> 
> "exist"? I don't know, to be honest. To me it makes no difference from
> hypervisor's point of view as intercepting tlb flushing instructions is
> not any different from implmenting a hypercall.
> 
> Hyper-V gives its guests 'hints' to indicate if they need to use
> hypercalls for remote/locat TLB flush and I don't remember seeing
> 'local' bit set.
> 
> Microsoft folks may probably shed some light on why this was added.

As Vitaly has indicated, these are based on hints from the hypervisor.
Not sure what the perf impact might be for the local flush enlightenment.
> 
> >
> >> +static void hyperv_flush_tlb_others(const struct cpumask *cpus,
> >> +				    struct mm_struct *mm, unsigned long
> start,
> >> +				    unsigned long end)
> >> +{
> >
> > What tree will this go through?  I'm about to send a signature change
> > for this function for tip:x86/mm.
> 
> I think this was going to get through Greg's char-misc tree but if we
> need to synchronize I think we can push this through x86.

It will be good to take this through Greg's tree as that would simplify coordination
with other changes. 
> 
> >
> > Also, how would this interact with PCID?  I have PCID patches that I'm
> > pretty happy with now, and I'm hoping to support PCID in 4.13.
> >
> 
> Sorry, I wasn't following this work closely. .flush_tlb_others() hook is
> not going away from pv_mmu_ops, right? In think case we can have both in
> 4.13. Or do you see any other clashes?
> 
> --
>   Vitaly
> _______________________________________________
> devel mailing list
> devel@linuxdriverproject.org
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> ev.linuxdriverproject.org%2Fmailman%2Flistinfo%2Fdriverdev-
> devel&data=02%7C01%7Ckys%40microsoft.com%7Cbdee6af479524fb02db50
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> 18wc%3D&reserved=0

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#1647247

On Mon, May 22, 2017 at 3:43 AM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> Andy Lutomirski <luto@kernel.org> writes:
>
>> On 05/19/2017 07:09 AM, Vitaly Kuznetsov wrote:
>>> Hyper-V host can suggest us to use hypercall for doing remote TLB flush,
>>> this is supposed to work faster than IPIs.
>>>
>>> Implementation details: to do HvFlushVirtualAddress{Space,List} hypercalls
>>> we need to put the input somewhere in memory and we don't really want to
>>> have memory allocation on each call so we pre-allocate per cpu memory areas
>>> on boot. These areas are of fixes size, limit them with an arbitrary number
>>> of 16 (16 gvas are able to specify 16 * 4096 pages).
>>>
>>> pv_ops patching is happening very early so we need to separate
>>> hyperv_setup_mmu_ops() and hyper_alloc_mmu().
>>>
>>> It is possible and easy to implement local TLB flushing too and there is
>>> even a hint for that. However, I don't see a room for optimization on the
>>> host side as both hypercall and native tlb flush will result in vmexit. The
>>> hint is also not set on modern Hyper-V versions.
>>
>> Why do local flushes exit?
>
> "exist"? I don't know, to be honest. To me it makes no difference from
> hypervisor's point of view as intercepting tlb flushing instructions is
> not any different from implmenting a hypercall.
>
> Hyper-V gives its guests 'hints' to indicate if they need to use
> hypercalls for remote/locat TLB flush and I don't remember seeing
> 'local' bit set.

What I meant was: why aren't local flushes handled directly in the
guest without exiting to the host?  Or are they?  In principle,
INVPCID should just work, right?  Even reading and writing CR3 back
should work if the hypervisor sets up the magic list of allowed CR3
values, right?

I guess on older CPUs there might not be any way to flush the local
TLB without exiting, but I'm not *that* familiar with the details of
the virtualization extensions.

>
>>
>>> +static void hyperv_flush_tlb_others(const struct cpumask *cpus,
>>> +                                struct mm_struct *mm, unsigned long start,
>>> +                                unsigned long end)
>>> +{
>>
>> What tree will this go through?  I'm about to send a signature change
>> for this function for tip:x86/mm.
>
> I think this was going to get through Greg's char-misc tree but if we
> need to synchronize I think we can push this through x86.

Works for me.  Linus can probably resolve the trivial conflict.  But
going through the x86 tree might make sense here if that's okay with
you.

>
>>
>> Also, how would this interact with PCID?  I have PCID patches that I'm
>> pretty happy with now, and I'm hoping to support PCID in 4.13.
>>
>
> Sorry, I wasn't following this work closely. .flush_tlb_others() hook is
> not going away from pv_mmu_ops, right? In think case we can have both in
> 4.13. Or do you see any other clashes?
>

The issue is that I'm changing the whole flush algorithm.  The main
patch that affects this is here:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/pcid&id=a67bff42e1e55666fdbaddf233a484a8773688c1

The interactions between that patch and paravirt flush helpers may be
complex, and it'll need some thought.  PCID makes everything even more
subtle, so just turning off PCID when paravirt flush is involved seems
the safest for now.  Ideally we'd eventually support PCID and paravirt
flushes together (and even eventual native remote flushes assuming
they ever get added).

Also, can you share the benchmark you used for these patches?

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#1647985

[Multipart message — attachments visible in raw view] — view raw

Andy Lutomirski <luto@kernel.org> writes:

> On Mon, May 22, 2017 at 3:43 AM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>> Andy Lutomirski <luto@kernel.org> writes:
>>
>>> On 05/19/2017 07:09 AM, Vitaly Kuznetsov wrote:
>>>> Hyper-V host can suggest us to use hypercall for doing remote TLB flush,
>>>> this is supposed to work faster than IPIs.
>>>>
>>>> Implementation details: to do HvFlushVirtualAddress{Space,List} hypercalls
>>>> we need to put the input somewhere in memory and we don't really want to
>>>> have memory allocation on each call so we pre-allocate per cpu memory areas
>>>> on boot. These areas are of fixes size, limit them with an arbitrary number
>>>> of 16 (16 gvas are able to specify 16 * 4096 pages).
>>>>
>>>> pv_ops patching is happening very early so we need to separate
>>>> hyperv_setup_mmu_ops() and hyper_alloc_mmu().
>>>>
>>>> It is possible and easy to implement local TLB flushing too and there is
>>>> even a hint for that. However, I don't see a room for optimization on the
>>>> host side as both hypercall and native tlb flush will result in vmexit. The
>>>> hint is also not set on modern Hyper-V versions.
>>>
>>> Why do local flushes exit?
>>
>> "exist"? I don't know, to be honest. To me it makes no difference from
>> hypervisor's point of view as intercepting tlb flushing instructions is
>> not any different from implmenting a hypercall.
>>
>> Hyper-V gives its guests 'hints' to indicate if they need to use
>> hypercalls for remote/locat TLB flush and I don't remember seeing
>> 'local' bit set.
>
> What I meant was: why aren't local flushes handled directly in the
> guest without exiting to the host?  Or are they?  In principle,
> INVPCID should just work, right?  Even reading and writing CR3 back
> should work if the hypervisor sets up the magic list of allowed CR3
> values, right?
>
> I guess on older CPUs there might not be any way to flush the local
> TLB without exiting, but I'm not *that* familiar with the details of
> the virtualization extensions.
>

Right, local flushes should 'just work'. If for whatever reason
hypervisor decides to trap us it's nothing we can do about it.

>>
>>>
>>>> +static void hyperv_flush_tlb_others(const struct cpumask *cpus,
>>>> +                                struct mm_struct *mm, unsigned long start,
>>>> +                                unsigned long end)
>>>> +{
>>>
>>> What tree will this go through?  I'm about to send a signature change
>>> for this function for tip:x86/mm.
>>
>> I think this was going to get through Greg's char-misc tree but if we
>> need to synchronize I think we can push this through x86.
>
> Works for me.  Linus can probably resolve the trivial conflict.  But
> going through the x86 tree might make sense here if that's okay with
> you.
>

Definitely fine with me, I'll leave this decision up to x86 maintainers,
Hyper-V maintainers, and Greg.

>>
>>>
>>> Also, how would this interact with PCID?  I have PCID patches that I'm
>>> pretty happy with now, and I'm hoping to support PCID in 4.13.
>>>
>>
>> Sorry, I wasn't following this work closely. .flush_tlb_others() hook is
>> not going away from pv_mmu_ops, right? In think case we can have both in
>> 4.13. Or do you see any other clashes?
>>
>
> The issue is that I'm changing the whole flush algorithm.  The main
> patch that affects this is here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/pcid&id=a67bff42e1e55666fdbaddf233a484a8773688c1
>
> The interactions between that patch and paravirt flush helpers may be
> complex, and it'll need some thought.  PCID makes everything even more
> subtle, so just turning off PCID when paravirt flush is involved seems
> the safest for now.  Ideally we'd eventually support PCID and paravirt
> flushes together (and even eventual native remote flushes assuming
> they ever get added).

I see. On Hyper-V HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST hypercall's
interface is:
1) List of entries to flush. Each entry is a PFN and lower 12 bits are
used to encode the number of pages after this one (defined by the PFN)
we'd like to flush. We can flush up to 509 entries with one
hypercall (can be extended but requires a pre-allocated memory region).

2) Processor mask

3) Address space id (all 64 bits of CR3. Not sure how it's used within
the hypervisor).

HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE_EX is more or less the same but we
need more space to specify > 64 vCPUs so we'll be able to pass less than
509 entries.

The main advantage compared to sending IPIs, as far as I understand, is
that virtual CPUs which are not currently scheduled don't need flushing
and we can't know this from within the guest.

I agree that disabling PCID for paravirt flush users for now is a good
option, let's have it merged and tested without this additional
complexity and make another round after.

>
> Also, can you share the benchmark you used for these patches?

I didn't do much while writing the patchset, mostly I was running the
attached dumb trasher (32 pthreads doing mmap/munmap). On a 16 vCPU
Hyper-V 2016 guest I get the following (just re-did the test with
4.12-rc1):

Before the patchset:
# time ./pthread_mmap ./randfile 

real	3m33.118s
user	0m3.698s
sys	3m16.624s

After the patchset:
# time ./pthread_mmap ./randfile 

real	2m19.920s
user	0m2.662s
sys	2m9.948s

K. Y.'s guys at Microsoft did additional testing for the patchset on
different Hyper-V deployments including Azure, they may share their
findings too.

-- 
  Vitaly

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#1648309

> -----Original Message-----
> From: devel [mailto:driverdev-devel-bounces@linuxdriverproject.org] On
> Behalf Of Vitaly Kuznetsov
> Sent: Tuesday, May 23, 2017 5:37 AM
> To: Andy Lutomirski <luto@kernel.org>
> Cc: Stephen Hemminger <sthemmin@microsoft.com>; Jork Loeser
> <Jork.Loeser@microsoft.com>; Haiyang Zhang <haiyangz@microsoft.com>;
> X86 ML <x86@kernel.org>; linux-kernel@vger.kernel.org; Steven Rostedt
> <rostedt@goodmis.org>; Ingo Molnar <mingo@redhat.com>; H. Peter Anvin
> <hpa@zytor.com>; devel@linuxdriverproject.org; Thomas Gleixner
> <tglx@linutronix.de>
> Subject: Re: [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB
> flush
> 
> Andy Lutomirski <luto@kernel.org> writes:
> 
> > On Mon, May 22, 2017 at 3:43 AM, Vitaly Kuznetsov
> <vkuznets@redhat.com> wrote:
> >> Andy Lutomirski <luto@kernel.org> writes:
> >>
> >>> On 05/19/2017 07:09 AM, Vitaly Kuznetsov wrote:
> >>>> Hyper-V host can suggest us to use hypercall for doing remote TLB
> flush,
> >>>> this is supposed to work faster than IPIs.
> >>>>
> >>>> Implementation details: to do HvFlushVirtualAddress{Space,List}
> hypercalls
> >>>> we need to put the input somewhere in memory and we don't really
> want to
> >>>> have memory allocation on each call so we pre-allocate per cpu
> memory areas
> >>>> on boot. These areas are of fixes size, limit them with an arbitrary
> number
> >>>> of 16 (16 gvas are able to specify 16 * 4096 pages).
> >>>>
> >>>> pv_ops patching is happening very early so we need to separate
> >>>> hyperv_setup_mmu_ops() and hyper_alloc_mmu().
> >>>>
> >>>> It is possible and easy to implement local TLB flushing too and there is
> >>>> even a hint for that. However, I don't see a room for optimization on
> the
> >>>> host side as both hypercall and native tlb flush will result in vmexit. The
> >>>> hint is also not set on modern Hyper-V versions.
> >>>
> >>> Why do local flushes exit?
> >>
> >> "exist"? I don't know, to be honest. To me it makes no difference from
> >> hypervisor's point of view as intercepting tlb flushing instructions is
> >> not any different from implmenting a hypercall.
> >>
> >> Hyper-V gives its guests 'hints' to indicate if they need to use
> >> hypercalls for remote/locat TLB flush and I don't remember seeing
> >> 'local' bit set.
> >
> > What I meant was: why aren't local flushes handled directly in the
> > guest without exiting to the host?  Or are they?  In principle,
> > INVPCID should just work, right?  Even reading and writing CR3 back
> > should work if the hypervisor sets up the magic list of allowed CR3
> > values, right?
> >
> > I guess on older CPUs there might not be any way to flush the local
> > TLB without exiting, but I'm not *that* familiar with the details of
> > the virtualization extensions.
> >
> 
> Right, local flushes should 'just work'. If for whatever reason
> hypervisor decides to trap us it's nothing we can do about it.
> 
> >>
> >>>
> >>>> +static void hyperv_flush_tlb_others(const struct cpumask *cpus,
> >>>> +                                struct mm_struct *mm, unsigned long start,
> >>>> +                                unsigned long end)
> >>>> +{
> >>>
> >>> What tree will this go through?  I'm about to send a signature change
> >>> for this function for tip:x86/mm.
> >>
> >> I think this was going to get through Greg's char-misc tree but if we
> >> need to synchronize I think we can push this through x86.
> >
> > Works for me.  Linus can probably resolve the trivial conflict.  But
> > going through the x86 tree might make sense here if that's okay with
> > you.
> >
> 
> Definitely fine with me, I'll leave this decision up to x86 maintainers,
> Hyper-V maintainers, and Greg.
> 
> >>
> >>>
> >>> Also, how would this interact with PCID?  I have PCID patches that I'm
> >>> pretty happy with now, and I'm hoping to support PCID in 4.13.
> >>>
> >>
> >> Sorry, I wasn't following this work closely. .flush_tlb_others() hook is
> >> not going away from pv_mmu_ops, right? In think case we can have both
> in
> >> 4.13. Or do you see any other clashes?
> >>
> >
> > The issue is that I'm changing the whole flush algorithm.  The main
> > patch that affects this is here:
> >
> >
> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.ker
> nel.org%2Fpub%2Fscm%2Flinux%2Fkernel%2Fgit%2Fluto%2Flinux.git%2Fco
> mmit%2F%3Fh%3Dx86%2Fpcid%26id%3Da67bff42e1e55666fdbaddf233a484a
> 8773688c1&data=02%7C01%7Ckys%40microsoft.com%7C88a812b285a741bcd
> 28d08d4a1d864aa%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636
> 311398154677248&sdata=%2BaCK2EW9S%2BdggL168xQ5eiaXXRZY31II6lLle1ys
> 6Bw%3D&reserved=0
> >
> > The interactions between that patch and paravirt flush helpers may be
> > complex, and it'll need some thought.  PCID makes everything even more
> > subtle, so just turning off PCID when paravirt flush is involved seems
> > the safest for now.  Ideally we'd eventually support PCID and paravirt
> > flushes together (and even eventual native remote flushes assuming
> > they ever get added).
> 
> I see. On Hyper-V HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST hypercall's
> interface is:
> 1) List of entries to flush. Each entry is a PFN and lower 12 bits are
> used to encode the number of pages after this one (defined by the PFN)
> we'd like to flush. We can flush up to 509 entries with one
> hypercall (can be extended but requires a pre-allocated memory region).
> 
> 2) Processor mask
> 
> 3) Address space id (all 64 bits of CR3. Not sure how it's used within
> the hypervisor).
> 
> HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE_EX is more or less the same but
> we
> need more space to specify > 64 vCPUs so we'll be able to pass less than
> 509 entries.
> 
> The main advantage compared to sending IPIs, as far as I understand, is
> that virtual CPUs which are not currently scheduled don't need flushing
> and we can't know this from within the guest.

There are other potential advantages as well:
1. When we need to flush with a large CPU mask, the hypercall mechanism can obviously
minimize the number of intercepts.
2. There is no instruction emulation in the hypercall path. 
> 
> I agree that disabling PCID for paravirt flush users for now is a good
> option, let's have it merged and tested without this additional
> complexity and make another round after.
> 
> >
> > Also, can you share the benchmark you used for these patches?
> 
> I didn't do much while writing the patchset, mostly I was running the
> attached dumb trasher (32 pthreads doing mmap/munmap). On a 16 vCPU
> Hyper-V 2016 guest I get the following (just re-did the test with
> 4.12-rc1):
> 
> Before the patchset:
> # time ./pthread_mmap ./randfile
> 
> real	3m33.118s
> user	0m3.698s
> sys	3m16.624s
> 
> After the patchset:
> # time ./pthread_mmap ./randfile
> 
> real	2m19.920s
> user	0m2.662s
> sys	2m9.948s
> 
> K. Y.'s guys at Microsoft did additional testing for the patchset on
> different Hyper-V deployments including Azure, they may share their
> findings too.

Our testing was mostly focused on stability and correctness. For the benchmarks we ran
(micro benchmarks for storage and networking) we did see improvements across the board.

Regards,

K. Y

> 
> --
>   Vitaly

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