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Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation

From Andrew Cooper <andrew.cooper3@citrix.com>
Newsgroups linux.kernel
Subject Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation
Date 2016-12-02 18:20 +0100
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On 02/12/16 17:07, Andy Lutomirski wrote:
> On Dec 2, 2016 3:44 AM, "Andrew Cooper" <andrew.cooper3@citrix.com> wrote:
>> On 02/12/16 00:35, Andy Lutomirski wrote:
>>> On Xen PV, CPUID is likely to trap, and Xen hypercalls aren't
>>> guaranteed to serialize.  (Even CPUID isn't *really* guaranteed to
>>> serialize on Xen PV, but, in practice, any trap it generates will
>>> serialize.)
>> Well, Xen will enabled CPUID Faulting wherever it can, which is
>> realistically all IvyBridge hardware and newer.
>>
>> All hypercalls are a privilege change to cpl0.  I'd hope this condition
>> is serialising, but I can't actually find any documentation proving or
>> disproving this.
> I don't know for sure.  IRET is serializing, and if Xen returns using
> IRET, we're fine.

All returns to a 64bit PV guest at defined points (hypercall return,
exception entry, etc) are from SYSRET, not IRET.

Talking of, I still have a patch to remove
PARAVIRT_ADJUST_EXCEPTION_FRAME which I need to complete and send upstream.

>
>>> On my laptop, CPUID(eax=1, ecx=0) is ~83ns and IRET-to-self is
>>> ~110ns.  But Xen PV will trap CPUID if possible, so IRET-to-self
>>> should end up being a nice speedup.
>>>
>>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>> CC'ing xen-devel and the Xen maintainers in Linux.
>>
>> As this is the only email from this series in my inbox, I will say this
>> here, but it should really be against patch 6.
>>
>> A write to %cr2 is apparently (http://sandpile.org/x86/coherent.htm) not
>> serialising on the 486, but I don't have a manual to hand to check.
> I'll quote the (modern) SDM.  For self-modifying code "The use of one
> of these options is not required for programs intended to run on the
> Pentium or Intel486 processors,
> but are recommended to ensure compatibility with the P6 and more
> recent processor families.".  For cross-modifying code "The use of
> this option is not required for programs intended to run on the
> Intel486 processor, but is recommended
> to ensure compatibility with the Pentium 4, Intel Xeon, P6 family, and
> Pentium processors."  So I'm not sure there's a problem.

Fair enough.  (Assuming similar properties hold for the older processors
of other vendors.)

~Andrew

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Thread

[PATCH 0/4] CPUID-less CPU fixes and improvements Andy Lutomirski <luto@kernel.org> - 2016-11-30 21:40 +0100
  [PATCH 4/4] x86/asm: Change sync_core() to use MOV to CR2 to serialize Andy Lutomirski <luto@kernel.org> - 2016-11-30 21:40 +0100
    Re: [PATCH 4/4] x86/asm: Change sync_core() to use MOV to CR2 to  serialize Borislav Petkov <bp@alien8.de> - 2016-12-01 10:30 +0100
      Re: [PATCH 4/4] x86/asm: Change sync_core() to use MOV to CR2 to serialize Andy Lutomirski <luto@amacapital.net> - 2016-12-01 18:10 +0100
        Re: [PATCH 4/4] x86/asm: Change sync_core() to use MOV to CR2 to  serialize Andrew Cooper <andrew.cooper3@citrix.com> - 2016-12-01 18:50 +0100
      Re: [PATCH 4/4] x86/asm: Change sync_core() to use MOV to CR2 to  serialize Ingo Molnar <mingo@kernel.org> - 2016-12-02 08:40 +0100
  [PATCH 3/4] x86/microcode/intel: Replace sync_core() with cpuid_eax(1) Andy Lutomirski <luto@kernel.org> - 2016-11-30 21:40 +0100
    Re: [PATCH 3/4] x86/microcode/intel: Replace sync_core() with  cpuid_eax(1) Borislav Petkov <bp@alien8.de> - 2016-12-01 10:20 +0100
  [PATCH v2 6/6] x86/asm: Change sync_core() to use MOV to CR2 to serialize Andy Lutomirski <luto@kernel.org> - 2016-12-02 01:40 +0100
  [PATCH v2 4/6] x86/paravirt: Make sync_core() be a paravirt op Andy Lutomirski <luto@kernel.org> - 2016-12-02 01:40 +0100
  [PATCH v2 3/6] x86/microcode/intel: Replace sync_core() with cpuid_eax(1) Andy Lutomirski <luto@kernel.org> - 2016-12-02 01:40 +0100
  [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Andy Lutomirski <luto@kernel.org> - 2016-12-02 01:40 +0100
    Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Andrew Cooper <andrew.cooper3@citrix.com> - 2016-12-02 12:50 +0100
      Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Andy Lutomirski <luto@amacapital.net> - 2016-12-02 18:10 +0100
        Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Andrew Cooper <andrew.cooper3@citrix.com> - 2016-12-02 18:20 +0100
          Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Andrew Cooper <andrew.cooper3@citrix.com> - 2016-12-02 18:30 +0100
          Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Andy Lutomirski <luto@amacapital.net> - 2016-12-02 18:30 +0100
      Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2016-12-02 19:50 +0100
        Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Andy Lutomirski <luto@amacapital.net> - 2016-12-02 20:40 +0100
      Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2016-12-02 21:10 +0100
    Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-02 18:40 +0100
      Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Andy Lutomirski <luto@kernel.org> - 2016-12-02 18:40 +0100
        Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Borislav Petkov <bp@kernel.org> - 2016-12-02 19:20 +0100
          Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-02 19:30 +0100
            Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Borislav Petkov <bp@kernel.org> - 2016-12-02 20:00 +0100
              Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-02 20:10 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-02 20:30 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Borislav Petkov <bp@alien8.de> - 2016-12-02 20:30 +0100
                [PATCH] x86/alternatives: Do not use sync_core() to serialize I$ Borislav Petkov <bp@alien8.de> - 2016-12-03 16:10 +0100
                Re: [PATCH] x86/alternatives: Do not use sync_core() to serialize I$ Andy Lutomirski <luto@amacapital.net> - 2016-12-03 18:20 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-02 20:40 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Andy Lutomirski <luto@amacapital.net> - 2016-12-02 21:50 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-02 22:20 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Andy Lutomirski <luto@amacapital.net> - 2016-12-03 00:00 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-03 00:20 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Andy Lutomirski <luto@amacapital.net> - 2016-12-02 20:40 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Andy Lutomirski <luto@amacapital.net> - 2016-12-02 20:30 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Borislav Petkov <bp@alien8.de> - 2016-12-02 20:40 +0100
                Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Borislav Petkov <bp@alien8.de> - 2016-12-02 20:30 +0100
        Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2016-12-02 19:20 +0100
        Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()  implementation Borislav Petkov <bp@alien8.de> - 2016-12-03 13:50 +0100
  [PATCH v2 2/6] Revert "x86/boot: Fail the boot if !M486 and CPUID is missing" Andy Lutomirski <luto@kernel.org> - 2016-12-02 01:40 +0100
  Re: [PATCH 0/4] CPUID-less CPU fixes and improvements Andy Lutomirski <luto@amacapital.net> - 2016-12-02 01:40 +0100
    Re: [PATCH 0/4] CPUID-less CPU fixes and improvements Ingo Molnar <mingo@kernel.org> - 2016-12-02 11:20 +0100
  [PATCH v2 0/6] CPUID-less CPU/sync_core fixes and improvements Andy Lutomirski <luto@kernel.org> - 2016-12-02 01:40 +0100
  [PATCH v2 1/6] x86/asm/32: Make sync_core() handle missing CPUID on all 32-bit kernels Andy Lutomirski <luto@kernel.org> - 2016-12-02 01:40 +0100

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