Re: [PATCH v3 9/11] KVM: MMU: prefetch ptes when intercepted guest #PF

Marcelo Tosatti
2010-07-01T12:08:23Z

On Thu, Jul 01, 2010 at 09:11:36AM +0800, Xiao Guangrong wrote:
>
>
> Marcelo Tosatti wrote:
>
> >> +
> >> + addr = gfn_to_hva_many(vcpu->kvm, gfn, &entry);
> >> + if (kvm_is_error_hva(addr))
> >> + return -1;
> >> +
> >> + entry = min(entry, (int)(end - start));
> >> + ret = __get_user_pages_fast(addr, entry, 1, pages);
> >> + if (ret <= 0)
> >> + return -1;
> >
> > Why can't you use gfn_to_pfn_atomic() here, one page at a time? Is
> > the overhead significant that this is worthwhile?
> >
> > You're bypassing the centralized interface.
>
> I think it's worthwhile to do since we can reduce gup overhead, no reason
> to traverse process's page table again and again for the consecutive pages.

I'd prefer to use gfn_to_pfn_atomic. The big win is with shadow mode and
there its already done one at a time.

> >
> >> +
> >> + pte_access = sp->role.access & FNAME(gpte_access)(vcpu, gpte);
> >> + mmu_set_spte(vcpu, spte, sp->role.access, pte_access, 0, 0,
> >> + is_dirty_gpte(gpte), NULL, sp->role.level, gfn,
> >> + pfn, true, false);
> >
> > reset_host_protection should be true, see commit 1403283acca (also for
> > direct case to be consistent).
> >
>
> Ah, i forgot it, thanks, Marcelo!
>
> I'll post a small fix patch base on this patchset to change 'false' to 'true' if no
> other places no to be improved. :-)