Re: [PATCH v2 4/6] KVM: x86/mmu: Fix wrong start gfn of tlb flushing with range

Hou Wenlong
2022-09-13T12:50:43Z

On Thu, Sep 08, 2022 at 02:25:42AM +0800, David Matlack wrote:
> On Wed, Aug 24, 2022 at 05:29:21PM +0800, Hou Wenlong wrote:
> > When a spte is dropped, the start gfn of tlb flushing should
> > be the gfn of spte not the base gfn of SP which contains the
> > spte. Also introduce a helper function to do range-based
> > flushing when a spte is dropped, which would help prevent
> > future buggy use of kvm_flush_remote_tlbs_with_address() in
> > such case.
> >
> > Fixes: c3134ce240eed ("KVM: Replace old tlb flush function with new one to flush a specified range.")
> > Suggested-by: David Matlack <dmatlack@google.com>
> > Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> > ---
> > arch/x86/kvm/mmu/mmu.c | 20 +++++++++++++++-----
> > arch/x86/kvm/mmu/paging_tmpl.h | 3 +--
> > 2 files changed, 16 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 3bcff56df109..e0b9432b9491 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -260,6 +260,18 @@ void kvm_flush_remote_tlbs_with_address(struct kvm *kvm,
> > kvm_flush_remote_tlbs_with_range(kvm, &range);
> > }
> >
> > +static gfn_t kvm_mmu_page_get_gfn(struct kvm_mmu_page *sp, int index);
> > +
> > +/* Flush the range of guest memory mapped by the given SPTE. */
> > +static void kvm_flush_remote_tlbs_sptep(struct kvm *kvm, u64 *sptep)
> > +{
> > + struct kvm_mmu_page *sp = sptep_to_sp(sptep);
> > + gfn_t gfn = kvm_mmu_page_get_gfn(sp, spte_index(sptep));
> > +
> > + kvm_flush_remote_tlbs_with_address(kvm, gfn,
> > + KVM_PAGES_PER_HPAGE(sp->role.level));
>
> How is the range-based TLB flushing supposed to work with indirect MMUs?
> When KVM is using shadow paging, the gfn here is not part of the actual
> translation.
>
> For example, when TDP is disabled, KVM's shadow page tables translate
> GVA to HPA. When Nested Virtualization is in use and running L2, KVM's
> shadow page tables translate nGPA to HPA.
>
> Ah, I see x86_ops.tlb_remote_flush_with_range is only set when running
> on Hyper-V and TDP is enabled (VMX checks enable_ept and SVM checks
> npt_enabled). But it looks like the nested case might still be broken?
>
Yeah, range based tlb flushing is only used on Hyper-V as Paolo said.
Actually, I don't know how Hyper-V implements the range based tlb
flushing hypercall, since KVM on Hyper-V is already nested, so gfn here
is already nGPA. It seems that the current used TDP root is passed in
hypercall, so maybe Hyper-V could do nGPA to HPA translation by looking
up TDP page table. Then for nested case, it chould work well?

> > +}
> > +
> > /* Flush all memory mapped by the given direct SP. */
> > static void kvm_flush_remote_tlbs_direct_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> > {
> > @@ -1156,8 +1168,7 @@ static void drop_large_spte(struct kvm *kvm, u64 *sptep, bool flush)
> > drop_spte(kvm, sptep);
> >
> > if (flush)
> > - kvm_flush_remote_tlbs_with_address(kvm, sp->gfn,
> > - KVM_PAGES_PER_HPAGE(sp->role.level));
> > + kvm_flush_remote_tlbs_sptep(kvm, sptep);
> > }
> >
> > /*
> > @@ -1608,7 +1619,7 @@ static void __rmap_add(struct kvm *kvm,
> > if (rmap_count > RMAP_RECYCLE_THRESHOLD) {
> > kvm_zap_all_rmap_sptes(kvm, rmap_head);
> > kvm_flush_remote_tlbs_with_address(
> > - kvm, sp->gfn, KVM_PAGES_PER_HPAGE(sp->role.level));
> > + kvm, gfn, KVM_PAGES_PER_HPAGE(sp->role.level));
> > }
> > }
> >
> > @@ -6402,8 +6413,7 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
> > kvm_zap_one_rmap_spte(kvm, rmap_head, sptep);
> >
> > if (kvm_available_flush_tlb_with_range())
> > - kvm_flush_remote_tlbs_with_address(kvm, sp->gfn,
> > - KVM_PAGES_PER_HPAGE(sp->role.level));
> > + kvm_flush_remote_tlbs_sptep(kvm, sptep);
> > else
> > need_tlb_flush = 1;
> >
> > diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> > index 39e0205e7300..04149c704d5b 100644
> > --- a/arch/x86/kvm/mmu/paging_tmpl.h
> > +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> > @@ -937,8 +937,7 @@ 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_with_address(vcpu->kvm,
> > - sp->gfn, KVM_PAGES_PER_HPAGE(sp->role.level));
> > + kvm_flush_remote_tlbs_sptep(vcpu->kvm, sptep);
> >
> > if (!rmap_can_add(vcpu))
> > break;
> > --
> > 2.31.1
> >