Re: [PATCH 4/4] KVM: MMU: Don't touch unsync sp in kvm_mmu_pte_write()

Avi Kivity
2010-09-26T13:09:14Z

On 09/23/2010 04:59 AM, Xiao Guangrong wrote:
> On 09/20/2010 11:24 PM, Avi Kivity wrote:
> > On 09/20/2010 04:21 PM, Xiao Guangrong wrote:
> >> Gfn may have many shadow pages, when one sp need be synced, we write
> >> protected sp->gfn and sync this sp but we keep other shadow pages
> >> asynchronous
> >>
> >> So, while gfn happen page fault, let it not touches unsync page, the
> >> unsync
> >> page only updated at invlpg/flush TLB time
> >>
> >> @@ -3157,6 +3164,9 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu,
> >> gpa_t gpa,
> >>
> >> mask.cr0_wp = mask.cr4_pae = mask.nxe = 1;
> >> for_each_gfn_indirect_valid_sp(vcpu->kvm, sp, gfn, node) {
> >> + if (sp->unsync)
> >> + continue;
> >> +
> >>
> >
> > Not sure this is a win. If a gpte is updated from having p=0 to p=1 (or
> > permissions upgraded), we may not have an invlpg to sync the spte, since
> > the hardware doesn't require it. With this change, we may get an extra
> > #PF.
> >
>
> Avi,
>
> Thanks for your review, i think this case is not too bad since:
>
> 1: This case only impacts local vcpu since if permissions is increased, it's
> no need send IPT to flush remote vcpu's tlb, so even if we update unsync
> spte in kvm_mmu_pte_write() path, the #PF still occur on other vcpus.

IIRC, the cpu will re-validate the tlb entry from the page tables before
issuing a fault, so we won't see a spurious fault. Not 100% sure.

For !P -> P, there won't be a tlb entry, so 100% there won't be a
spurious fault.

> 2: If the unsync sp which is updated in kvm_mmu_pte_write() is not using by the
> vcpu, it will sync automatically after it's loaded.

True, and this is a likely case.

> 3: If the sp is using, update this sp in kvm_mmu_pte_write() will avoid extra #PF,
> in this case, two(or more) sps have the same gfn, there are mapped in the same
> page table and with different kinds(unsync/sync), i thinks this case is infrequency.
> And even we updated it, we can not sure it can be accessed latter,

If it's infrequent, the why do we optimize it?

> So, i think it's better lazily update unsync sp until it's used or the flush time,
> your opinion? :-)
>

Any performance numbers?

To me it seems saving a possible exit is worth extra computation.

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