linux-kernel - Re: [PATCH 19/27] KVM: x86/mmu: Use page-track notifiers iff there are external users

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Date:   Thu, 10 Aug 2023 08:41:14 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     Like Xu <like.xu.linux@...il.com>, kvm@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Ben Gardon <bgardon@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        intel-gvt-dev@...ts.freedesktop.org,
        Zhi Wang <zhi.a.wang@...el.com>
Subject: Re: [PATCH 19/27] KVM: x86/mmu: Use page-track notifiers iff there
 are external users

On Thu, Aug 10, 2023, Yan Zhao wrote:
> On Thu, Aug 10, 2023 at 07:21:03AM +0800, Yan Zhao wrote:
> > > Reading the value after acquiring mmu_lock ensures that both vCPUs will see whatever
> > > value "loses" the race, i.e. whatever written value is processed second ('Y' in the
> > > above sequence).
> > I suspect that vCPU0 may still generate a wrong SPTE if vCPU1 wrote 4
> > bytes while vCPU0 wrote 8 bytes, though the chances are very low.
> > 
> This could happen in below sequence:
> vCPU0 updates a PTE to AABBCCDD;
> vCPU1 updates a PTE to EEFFGGHH in two writes.
> (each character stands for a byte)
> 
> vCPU0                  vCPU1   
> write AABBCCDD
>                        write GGHH
>                        detect 4 bytes write and hold on sync
> sync SPTE w/ AABBGGHH
>                        write EEFF
>                        sync SPTE w/ EEFFGGHH
> 
> 
> Do you think it worth below serialization work?

No, because I don't see any KVM bugs with the above sequence.  If the guest doesn't
ensure *all* writes from vCPU0 and vCPU1 are fully serialized, then it is completely
legal for hardware (KVM in this case) to consume AABBGGHH as a PTE.  The only thing
the guest shouldn't see is EEFFCCDD, but I don't see how that can happen.