linux-kernel - Re: [PATCH 2/2] KVM: x86/mmu: Retry fault before acquiring mmu

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Date:   Thu, 7 Sep 2023 22:34:49 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: Re: [PATCH 2/2] KVM: x86/mmu: Retry fault before acquiring mmu_lock
 if mapping is changing

On Thu, 2023-09-07 at 07:45 -0700, Sean Christopherson wrote:
> On Wed, Sep 06, 2023, Kai Huang wrote:
> > On Thu, 2023-08-24 at 19:07 -0700, Sean Christopherson wrote:
> > > ---
> > >  arch/x86/kvm/mmu/mmu.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 1a5a1e7d1eb7..8e2e07ed1a1b 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -4334,6 +4334,9 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> > >  	if (unlikely(!fault->slot))
> > >  		return kvm_handle_noslot_fault(vcpu, fault, access);
> > >  
> > > +	if (mmu_invalidate_retry_hva(vcpu->kvm, fault->mmu_seq, fault->hva))
> > > +		return RET_PF_RETRY;
> > > +
> > 
> > ... Perhaps a comment saying this is to avoid unnecessary MMU lock contention
> > would be nice.  Otherwise we have is_page_fault_stale() called later within the
> > MMU lock.  I suppose people only tend to use git blamer when they cannot find
> > answer in the code :-)
> 
> Agreed, will add.
> 
> > >  	return RET_PF_CONTINUE;
> > >  }
> > >  
> > 
> > Btw, currently fault->mmu_seq is set in kvm_faultin_pfn(), which happens after
> > fast_page_fault().  Conceptually, should we move this to even before
> > fast_page_fault() because I assume the range zapping should also apply to the
> > cases that fast_page_fault() handles?
> 
> Nope, fast_page_fault() doesn't need to "manually" detect invalidated SPTEs because
> it only modifies shadow-present SPTEs and does so with an atomic CMPXCHG.  If a
> SPTE is zapped by an mmu_notifier event (or anything else), the CMPXCHG will fail
> and fast_page_fault() will see the !PRESENT SPTE on the next retry and bail.

Ah yes.  Thanks.