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    SubjectRe: [PATCH v2 8/10] KVM: MMU: prefetch ptes when intercepted guest #PF
    On Tue, Jun 29, 2010 at 04:07:40PM +0800, Xiao Guangrong wrote:
    >
    >
    > Marcelo Tosatti wrote:
    >
    > >> +
    > >> + if (sp->role.level > PT_PAGE_TABLE_LEVEL)
    > >> + return;
    > >> +
    > >> + if (sp->role.direct)
    > >> + return direct_pte_prefetch(vcpu, sptep);
    > >
    > > Can never happen.
    > >
    >
    > Marcelo,
    >
    > Thanks for your comment. You mean that we can't meet sp->role.direct here?
    > could you please tell me why? During my test, it can be triggered.

    Ah, for 1->1 emulation, right.

    > >> @@ -322,6 +395,7 @@ static u64 *FNAME(fetch)(struct kvm_vcpu *vcpu, gva_t addr,
    > >> user_fault, write_fault,
    > >> dirty, ptwrite, level,
    > >> gw->gfn, pfn, false, true);
    > >> + FNAME(pte_prefetch)(vcpu, sptep);
    > >> break;
    > >> }
    > >
    > >
    > > I'm afraid this can introduce regressions since it increases mmu_lock
    > > contention. Can you get some numbers with 4-vcpu or 8-vcpu guest and
    > > many threads benchmarks, such as kernbench and apachebench? (on
    > > non-EPT).
    > >
    >
    > The pte prefetch is the fast path, it only occupies little time, for the worst
    > case, only need read 128 byte form the guest pte, and if it prefetched success,
    > the #PF cause by later access will avoid, then we avoid to exit form the guest,
    > and walk guest pte, walk shadow pages, flush local tlb... a lots of work can be
    > reduced.
    >
    > Before i post this patchset firstly, i do the performance test by using unixbench,
    > it improved ~3.6% under EPT disable case.
    > (it's in the first version's chagelog)
    >
    > Today, i do the kernbench test with 4 vcpu and 1G memory, the result shows it
    > improved ~1.6% :-)

    OK, nice.

    > > Also prefetch should be disabled for EPT, due to lack of accessed bit.
    > >
    >
    > But we call mmu_set_spte() with speculative == false, it not touch the accessed bit.

    There is no accessed bit on EPT. So the aging code (kvm_age_rmapp)
    considers any present translation as accessed. There is no way to
    distinguish between actually accessed translations and prefetched (but
    unused) ones.


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