Re: [PATCH v3 6/6] KVM: MMU: delay flush all tlbs on sync_page path

Marcelo Tosatti
2010-11-22T21:46:24Z

On Mon, Nov 22, 2010 at 12:19:25PM -0200, Marcelo Tosatti wrote:
> On Mon, Nov 22, 2010 at 11:45:18AM +0800, Xiao Guangrong wrote:
> > On 11/20/2010 12:11 AM, Marcelo Tosatti wrote:
> >
> > >> void kvm_flush_remote_tlbs(struct kvm *kvm)
> > >> {
> > >> + int dirty_count = atomic_read(&kvm->tlbs_dirty);
> > >> +
> > >> + smp_mb();
> > >> if (make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
> > >> ++kvm->stat.remote_tlb_flush;
> > >> + atomic_sub(dirty_count, &kvm->tlbs_dirty);
> > >> }
> > >
> > > This is racy because kvm_flush_remote_tlbs might be called without
> > > mmu_lock protection.
> >
> > Sorry for my carelessness, it should be 'cmpxchg' here.
> >
> > > You could decrease the counter on
> > > invalidate_page/invalidate_range_start only,
> >
> > I want to avoid a unnecessary tlbs flush, if tlbs have been flushed
> > after sync_page, then we don't need flush tlbs on invalidate_page/
> > invalidate_range_start path.
> >
> > > these are not fast paths
> > > anyway.
> > >
> >
> > How about below patch? it just needs one atomic operation.
> >
> > ---
> > arch/x86/kvm/paging_tmpl.h | 4 ++--
> > include/linux/kvm_host.h | 2 ++
> > virt/kvm/kvm_main.c | 7 ++++++-
> > 3 files changed, 10 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> > index dfb906f..e64192f 100644
> > --- a/arch/x86/kvm/paging_tmpl.h
> > +++ b/arch/x86/kvm/paging_tmpl.h
> > @@ -781,14 +781,14 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
> > gfn = gpte_to_gfn(gpte);
> >
> > if (FNAME(map_invalid_gpte)(vcpu, sp, &sp->spt[i], gpte)) {
> > - kvm_flush_remote_tlbs(vcpu->kvm);
> > + vcpu->kvm->tlbs_dirty++;
> > continue;
> > }
> >
> > if (gfn != sp->gfns[i]) {
> > drop_spte(vcpu->kvm, &sp->spt[i],
> > shadow_trap_nonpresent_pte);
> > - kvm_flush_remote_tlbs(vcpu->kvm);
> > + vcpu->kvm->tlbs_dirty++;
> > continue;
> > }
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 4bd663d..dafd90e 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -249,6 +249,7 @@ struct kvm {
> > struct mmu_notifier mmu_notifier;
> > unsigned long mmu_notifier_seq;
> > long mmu_notifier_count;
> > + long tlbs_dirty;
> > #endif
> > };
> >
> > @@ -377,6 +378,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *vcpu);
> > void kvm_resched(struct kvm_vcpu *vcpu);
> > void kvm_load_guest_fpu(struct kvm_vcpu *vcpu);
> > void kvm_put_guest_fpu(struct kvm_vcpu *vcpu);
> > +
> > void kvm_flush_remote_tlbs(struct kvm *kvm);
> > void kvm_reload_remote_mmus(struct kvm *kvm);
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index fb93ff9..fe0a1a7 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -168,8 +168,12 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
> >
> > void kvm_flush_remote_tlbs(struct kvm *kvm)
> > {
> > + long dirty_count = kvm->tlbs_dirty;
> > +
> > + smp_mb();
> > if (make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
> > ++kvm->stat.remote_tlb_flush;
>
> <---
>
> > + cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);
> > }
>
>
> Still problematic, if tlbs_dirty is set in the point indicated above.
>
> Invalidate page should be quite rare, so checking for tlb_dirty only
> there is OK.
>

My bad, the patch is fine.