linux-kernel - Re: [PATCH v4 0/5] KVM: x86: flush tlb out of mmu-lock after write protection

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Message-ID: <CAHqbYQskGiDtmj_OmRnYn-ueu9GAfkNZvh1VX68oV1rzBrv8wg@mail.gmail.com>
Date:	Wed, 26 Mar 2014 00:40:04 -0400
From:	Hu Yaohui <loki2441@...il.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Gleb Natapov <gleb@...nel.org>, Avi Kivity <avi.kivity@...il.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH v4 0/5] KVM: x86: flush tlb out of mmu-lock after write protection

Hi all,
I hope you have a good day!
I have debugged the code myself. I have called dump_stack() in
function "__kvm_unsync_page"
and function "invlpg".  Actually every time before invlpg is called,
the page fault handled will call "__kvm_unsync_page" before invlpg to
mark the specified sp as unsynced. (correct me if I am wrong). I am
wondering why there is a page fault. AFAIK when calling flush_tlb_page
in the guest os. it will issue invlpg instruction directly, I did not
see any operation which could always cause the page fault.I would
really appreciate if if someone could shed me some lights on it.
Thanks for your time!

Best Wishes,
Yaohui

On Tue, Mar 25, 2014 at 12:25 PM, Hu Yaohui <loki2441@...il.com> wrote:
> Hi Guangrong,
> Since you have written in the kvm/mmu.txt.
> <quote>
>
>   unsync:
>     If true, then the translations in this page may not match the guest's
>     translation.  This is equivalent to the state of the tlb when a pte is
>     changed but before the tlb entry is flushed.  Accordingly, unsync ptes
>     are synchronized when the guest executes invlpg or flushes its tlb by
>     other means.  Valid for leaf pages.
>
> </quote>
>
> This make sense to me, my question is when those unsync bits will be
> set? When the guest writes to the level 1 guest page tables, it will
> not cause a page fault. Those unsync bit is unlikely to be set when
> the entry is modified. (correct me if I am wrong).
>
> <code>
> static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva)
> {
> ...
>     for_each_shadow_entry(vcpu, gva, iterator) {
>         level = iterator.level;
>         sptep = iterator.sptep;
>
>         sp = page_header(__pa(sptep));
>         if (is_last_spte(*sptep, level)) {
>             int offset, shift;
>
>             if (!sp->unsync)
>                 break;
> ...
> </code>
> When guest called invlpg, kvm invlpg will navigate to to the last
> level,  if the sp->unsync is not set to 1, since the initial value is
> zero. it will just break. It's not straight forward to me that the
> specified sp will be synced with the guest page table.
>
> I think I have missed something or misunderstood the whole mechanism,
> I would really appreciate it if you could shed some lights on that.
>
> Best Wishes,
> Yaohui Hu
>
> On Mon, Mar 10, 2014 at 10:01 AM, Xiao Guangrong
> <xiaoguangrong@...ux.vnet.ibm.com> wrote:
>> This patchset is splited from my previous patchset:
>> [PATCH v3 00/15] KVM: MMU: locklessly write-protect
>> that can be found at:
>> https://lkml.org/lkml/2013/10/23/265
>>
>> Changelog v4 :
>> - add more comments for patch 5 and thank for Marcelo's review
>>
>> Xiao Guangrong (5):
>>   Revert "KVM: Simplify kvm->tlbs_dirty handling"
>>   KVM: MMU: properly check last spte in fast_page_fault()
>>   KVM: MMU: lazily drop large spte
>>   KVM: MMU: flush tlb if the spte can be locklessly modified
>>   KVM: MMU: flush tlb out of mmu lock when write-protect the sptes
>>
>>  arch/x86/kvm/mmu.c         | 72 ++++++++++++++++++++++++++++++----------------
>>  arch/x86/kvm/mmu.h         | 14 +++++++++
>>  arch/x86/kvm/paging_tmpl.h |  7 ++---
>>  arch/x86/kvm/x86.c         | 20 ++++++++++---
>>  include/linux/kvm_host.h   |  4 +--
>>  virt/kvm/kvm_main.c        |  5 +++-
>>  6 files changed, 85 insertions(+), 37 deletions(-)
>>
>> --
>> 1.8.1.4
>>
>> --
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