linux-kernel - Re: [PATCH v2 6/6] KVM: x86/mmu: Use 1 as the size of gfn range for tlb flushing in FNAME(invlpg)()

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Date:   Fri, 16 Sep 2022 19:33:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     Hou Wenlong <houwenlong.hwl@...group.com>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 6/6] KVM: x86/mmu: Use 1 as the size of gfn range for
 tlb flushing in FNAME(invlpg)()

On Tue, Sep 13, 2022, David Matlack wrote:
> On Tue, Sep 13, 2022 at 5:58 AM Hou Wenlong <houwenlong.hwl@...group.com> wrote:
> >
> > On Thu, Sep 08, 2022 at 01:40:16AM +0800, David Matlack wrote:
> > > On Wed, Aug 24, 2022 at 05:29:23PM +0800, Hou Wenlong wrote:
> > > > Only SP with PG_LEVLE_4K level could be unsync, so the size of gfn range
> > > > must be 1.
> > > >
> > > > Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
> > > > ---
> > > >  arch/x86/kvm/mmu/paging_tmpl.h | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> > > > index 04149c704d5b..486a3163b1e4 100644
> > > > --- a/arch/x86/kvm/mmu/paging_tmpl.h
> > > > +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> > > > @@ -937,7 +937,8 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa)
> > > >
> > > >                     mmu_page_zap_pte(vcpu->kvm, sp, sptep, NULL);
> > > >                     if (is_shadow_present_pte(old_spte))
> > > > -                           kvm_flush_remote_tlbs_sptep(vcpu->kvm, sptep);
> > > > +                           kvm_flush_remote_tlbs_gfn(vcpu->kvm,
> > > > +                                   kvm_mmu_page_get_gfn(sp, sptep - sp->spt), 1);
> > >
> > > The third argument to kvm_flush_remote_tlbs_gfn() is the level, not the
> > > number of pages. But that aside, I don't understand why this patch is
> > > necessary. kvm_flush_remote_tlbs_sptep() should already do the right
> > > thing.
> > >
> > Since only SP with PG_LEVEL_4K level could be unsync, so the level must
> > be PG_LEVEL_4K, then sp->role.level access could be dropped. However,
> > I'm not sure whether it is useful. I can drop it if it is useless.
> 
> Ah, I see. I would be surprised if avoiding the read of sp->role.level
> has any noticeable impact on VM performance so I vote to drop this patch.

Agreed, the cost of the sp->role.level lookup is negligible in this case, and IMO
using kvm_flush_remote_tlbs_sptep() is more intuitive.

If kvm_flush_remote_tlbs_sptep() didn't exist and this was open coding the use of
kvm_flush_remote_tlbs_with_address() + KVM_PAGES_PER_HPAGE(), then I would be in
favor of hardcoding '1', because at that point the use of KVM_PAGES_PER_HPAGE() is
misleading in its own way.