Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush
Date	2017-08-11 15:50 +0200
Message-ID	<udiu5-4b4-3@gated-at.bofh.it> (permalink)
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On Fri, Aug 11, 2017 at 03:07:29PM +0200, Juergen Gross wrote:
> On 11/08/17 14:54, Peter Zijlstra wrote:
> > On Fri, Aug 11, 2017 at 02:46:41PM +0200, Juergen Gross wrote:
> >> Aah, okay. Now I understand the problem. The TLB isn't the issue but the
> >> IPI is serving two purposes here: TLB flushing (which is allowed to
> >> happen at any time) and serialization regarding access to critical pages
> >> (which seems to be broken in the Xen case as you suggest).
> > 
> > Indeed, and now hyper-v as well.
> 
> Is it possible to distinguish between non-critical calls of
> flush_tlb_others() (which should be the majority IMHO) and critical ones
> regarding above problem? I guess the only problem is the case when a
> page table can be freed because its last valid entry is gone, right?
> 
> We might want to add a serialization flag to indicate flushing _and_
> serialization via IPI should be performed.

Possible, but not trivial. Esp things like transparent huge pages, which
swizzles PMDs around makes things tricky.

The by far easiest solution is to switch over to HAVE_RCU_TABLE_FREE
when either Xen or Hyper-V is doing this. Ideally it would not have a
significant performance hit (needs testing) and we can simply always do
this when PARAVIRT, or otherwise we need to get creative with
static_keys or something.

Thread

[PATCH v10 0/9] Hyper-V: paravirtualized remote TLB flushing and hypercall improvements Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-02 18:10 +0200
  [PATCH v10 1/9] x86/hyper-v: include hyperv/ only when CONFIG_HYPERV is set Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-02 18:10 +0200
    [tip:x86/platform] x86/hyper-v: Include hyperv/ only when  CONFIG_HYPERV is set tip-bot for Vitaly Kuznetsov <tipbot@zytor.com> - 2017-08-10 18:50 +0200
  [PATCH v10 3/9] x86/hyper-v: fast hypercall implementation Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-02 18:10 +0200
    [tip:x86/platform] x86/hyper-v: Introduce fast hypercall  implementation tip-bot for Vitaly Kuznetsov <tipbot@zytor.com> - 2017-08-10 18:50 +0200
  [PATCH v10 7/9] x86/hyper-v: use hypercall for remote TLB flush Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-02 18:20 +0200
    [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush tip-bot for Vitaly Kuznetsov <tipbot@zytor.com> - 2017-08-10 18:50 +0200
    [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush tip-bot for Vitaly Kuznetsov <tipbot@zytor.com> - 2017-08-10 20:30 +0200
      Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Peter Zijlstra <peterz@infradead.org> - 2017-08-10 21:00 +0200
        Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Peter Zijlstra <peterz@infradead.org> - 2017-08-10 21:30 +0200
          Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Peter Zijlstra <peterz@infradead.org> - 2017-08-11 11:10 +0200
            Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush "Kirill A. Shutemov" <kirill@shutemov.name> - 2017-08-11 13:30 +0200
            Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-11 18:20 +0200
              Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Peter Zijlstra <peterz@infradead.org> - 2017-08-11 18:30 +0200
                Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-14 15:30 +0200
                Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-16 18:50 +0200
                Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-08-16 23:50 +0200
                Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-17 10:00 +0200
          Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-11 11:30 +0200
            Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Peter Zijlstra <peterz@infradead.org> - 2017-08-11 13:00 +0200
              Re: [Xen-devel] [tip:x86/platform] x86/hyper-v: Use hypercall for  remote TLB flush Andrew Cooper <andrew.cooper3@citrix.com> - 2017-08-11 13:10 +0200
                Re: [Xen-devel] [tip:x86/platform] x86/hyper-v: Use hypercall for  remote TLB flush Peter Zijlstra <peterz@infradead.org> - 2017-08-11 14:10 +0200
                Re: [Xen-devel] [tip:x86/platform] x86/hyper-v: Use hypercall for  remote TLB flush Steven Rostedt <rostedt@goodmis.org> - 2017-08-16 02:10 +0200
              Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Juergen Gross <jgross@suse.com> - 2017-08-11 14:30 +0200
                Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Peter Zijlstra <peterz@infradead.org> - 2017-08-11 14:40 +0200
                Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Juergen Gross <jgross@suse.com> - 2017-08-11 14:50 +0200
                Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Peter Zijlstra <peterz@infradead.org> - 2017-08-11 15:00 +0200
                Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Juergen Gross <jgross@suse.com> - 2017-08-11 15:10 +0200
                Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB  flush Peter Zijlstra <peterz@infradead.org> - 2017-08-11 15:50 +0200
  [PATCH v10 4/9] hyper-v: use fast hypercall for HVCALL_SIGNAL_EVENT Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-02 18:20 +0200
    [tip:x86/platform] hyper-v: Use fast hypercall for  HVCALL_SIGNAL_EVENT tip-bot for Vitaly Kuznetsov <tipbot@zytor.com> - 2017-08-10 18:50 +0200
  [PATCH v10 5/9] x86/hyper-v: implement rep hypercalls Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-02 18:20 +0200
    [tip:x86/platform] x86/hyper-v: Implement rep hypercalls tip-bot for Vitaly Kuznetsov <tipbot@zytor.com> - 2017-08-10 18:50 +0200
  [PATCH v10 8/9] x86/hyper-v: support extended CPU ranges for TLB flush hypercalls Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-02 18:20 +0200
  [PATCH v10 9/9] tracing/hyper-v: trace hyperv_mmu_flush_tlb_others() Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-02 18:20 +0200
  [PATCH v10 6/9] hyper-v: globalize vp_index Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-02 18:20 +0200
    [tip:x86/platform] hyper-v: Globalize vp_index tip-bot for Vitaly Kuznetsov <tipbot@zytor.com> - 2017-08-10 18:50 +0200
  Re: [PATCH v10 0/9] Hyper-V: paravirtualized remote TLB flushing and hypercall improvements Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-10 14:00 +0200
    Re: [PATCH v10 0/9] Hyper-V: paravirtualized remote TLB flushing and  hypercall improvements Ingo Molnar <mingo@kernel.org> - 2017-08-10 17:20 +0200
      Re: [PATCH v10 0/9] Hyper-V: paravirtualized remote TLB flushing and hypercall improvements Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-10 17:20 +0200
  Re: [PATCH v10 0/9] Hyper-V: paravirtualized remote TLB flushing and hypercall improvements Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-08-10 19:10 +0200

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