Re: [PATCH 7/7] KVM: X86: Also prefetch the last range in __direct_pte_prefetch().

[Lai Jiangshan]

On Wed, Aug 25, 2021 at 11:18 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Tue, Aug 24, 2021, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> >
> > __direct_pte_prefetch() skips prefetching the last range.
> >
> > The last range are often the whole range after the faulted spte when
> > guest is touching huge-page-mapped(in guest view) memory forwardly
> > which means prefetching them can reduce pagefault.
> >
> > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index e5932af6f11c..ac260e01e9d8 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -2847,8 +2847,9 @@ static void __direct_pte_prefetch(struct kvm_vcpu *vcpu,
> >       i = (sptep - sp->spt) & ~(PTE_PREFETCH_NUM - 1);
> >       spte = sp->spt + i;
> >
> > -     for (i = 0; i < PTE_PREFETCH_NUM; i++, spte++) {
> > -             if (is_shadow_present_pte(*spte) || spte == sptep) {
> > +     for (i = 0; i <= PTE_PREFETCH_NUM; i++, spte++) {
> > +             if (i == PTE_PREFETCH_NUM ||
> > +                 is_shadow_present_pte(*spte) || spte == sptep) {
>
> Heh, I posted a fix just a few days ago.  I prefer having a separate call after
> the loop.  The "<= PTE_PREFETCH_NUM" is subtle, and a check at the ends avoids
> a CMP+Jcc in the loop, though I highly doubt that actually affects performance.
>
> https://lkml.kernel.org/r/20210818235615.2047588-1-seanjc@xxxxxxxxxx

Thanks!

>
> >                       if (!start)
> >                               continue;
> >                       if (direct_pte_prefetch_many(vcpu, sp, start, spte) < 0)
> > --
> > 2.19.1.6.gb485710b
> >