On Mon, Jun 05, 2023, Mingwei Zhang wrote:
> On Mon, Jun 5, 2023 at 9:55 AM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
> >
> > On Sun, Jun 4, 2023 at 5:43 PM Mingwei Zhang <mizhang@xxxxxxxxxx> wrote:
> > >
> > > Remove KVM MMU write lock when accessing indirect_shadow_pages counter when
> > > page role is direct because this counter value is used as a coarse-grained
> > > heuristics to check if there is nested guest active. Racing with this
> > > heuristics without mmu lock will be harmless because the corresponding
> > > indirect shadow sptes for the GPA will either be zapped by this thread or
> > > some other thread who has previously zapped all indirect shadow pages and
> > > makes the value to 0.
> > >
> > > Because of that, remove the KVM MMU write lock pair to potentially reduce
> > > the lock contension and improve the performance of nested VM. In addition
> > > opportunistically change the comment of 'direct mmu' to make the
> > > description consistent with other places.
> > >
> > > Reported-by: Jim Mattson <jmattson@xxxxxxxxxx>
> > > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > > ---
> > > arch/x86/kvm/x86.c | 10 ++--------
> > > 1 file changed, 2 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 5ad55ef71433..97cfa5a00ff2 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -8585,15 +8585,9 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> > >
> > > kvm_release_pfn_clean(pfn);
> > >
> > > - /* The instructions are well-emulated on direct mmu. */
> > > + /* The instructions are well-emulated on Direct MMUs. */
> > > if (vcpu->arch.mmu->root_role.direct) {
> > > - unsigned int indirect_shadow_pages;
> > > -
> > > - write_lock(&vcpu->kvm->mmu_lock);
> > > - indirect_shadow_pages = vcpu->kvm->arch.indirect_shadow_pages;
> > > - write_unlock(&vcpu->kvm->mmu_lock);
> > > -
> > > - if (indirect_shadow_pages)
> > > + if (READ_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
> >
> > I don't understand the need for READ_ONCE() here. That implies that
> > there is something tricky going on, and I don't think that's the case.
>
> READ_ONCE() is just telling the compiler not to remove the read. Since
> this is reading a global variable, the compiler might just read a
> previous copy if the value has already been read into a local
> variable. But that is not the case here...
>
> Note I see there is another READ_ONCE for
> kvm->arch.indirect_shadow_pages, so I am reusing the same thing.
I agree with Jim, using READ_ONCE() doesn't make any sense. I suspect it may have
been a misguided attempt to force the memory read to be as close to the write_lock()
as possible, e.g. to minimize the chance of a false negative.
> I did check the reordering issue but it should be fine because when
> 'we' see indirect_shadow_pages as 0, the shadow pages must have
> already been zapped. Not only because of the locking, but also the
> program order in __kvm_mmu_prepare_zap_page() shows that it will zap
> shadow pages first before updating the stats.
I don't think zapping, i.e. the 1=>0 transition, is a concern. KVM is dropping
the SPTE, so racing with kvm_mmu_pte_write() is a non-issue because the guest
will either see the old value, or will fault after the SPTE is zapped, i.e. KVM
won't run with a stale even if kvm_mmu_pte_write() sees '0' before TLBs are
flushed.
I believe the 0=>1 transition on the other hand doesn't have a *very* theoretical
bug. KVM needs to ensure that either kvm_mmu_pte_write() sees an elevated count,
or that a page fault task sees the updated guest PTE, i.e. the emulated write.
The READ_ONCE() likely serves this purpose in practice, though technically it's
insufficient.
So I think this?
---
arch/x86/kvm/mmu.h | 14 ++++++++++++++
arch/x86/kvm/mmu/mmu.c | 13 ++++++++++++-
arch/x86/kvm/x86.c | 8 +-------
3 files changed, 27 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 92d5a1924fc1..9cd105ccb1d4 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -264,6 +264,20 @@ static inline bool kvm_memslots_have_rmaps(struct kvm *kvm)
return !tdp_mmu_enabled || kvm_shadow_root_allocated(kvm);
}
+static inline bool kvm_mmu_has_indirect_shadow_pages(struct kvm *kvm)
+{
+ /*
+ * When emulating guest writes, ensure the written value is visible to
+ * any task that is handling page faults before checking whether or not
+ * KVM is shadowing a guest PTE. This ensures either KVM will create
+ * the correct SPTE in the page fault handler, or this task will see
+ * a non-zero indirect_shadow_pages. Pairs with the smp_mb() in
+ * account_shadowed() and unaccount_shadowed().
+ */
+ smp_mb();
+ return kvm->arch.indirect_shadow_pages;
+}
+
static inline gfn_t gfn_to_index(gfn_t gfn, gfn_t base_gfn, int level)
{
/* KVM_HPAGE_GFN_SHIFT(PG_LEVEL_4K) must be 0. */
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index c8961f45e3b1..1735bee3f653 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -830,6 +830,17 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
gfn_t gfn;
kvm->arch.indirect_shadow_pages++;
+
+ /*
+ * Ensure indirect_shadow_pages is elevated prior to re-reading guest
+ * child PTEs in FNAME(gpte_changed), i.e. guarantee either in-flight
+ * emulated writes are visible before re-reading guest PTEs, or that
+ * an emulated write will see the elevated count and acquire mmu_lock
+ * to update SPTEs. Pairs with the smp_mb() in
+ * kvm_mmu_has_indirect_shadow_pages().
+ */
+ smp_mb();
+
gfn = sp->gfn;
slots = kvm_memslots_for_spte_role(kvm, sp->role);
slot = __gfn_to_memslot(slots, gfn);
@@ -5692,7 +5703,7 @@ static void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
* If we don't have indirect shadow pages, it means no page is
* write-protected, so we can exit simply.
*/
- if (!READ_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
+ if (!kvm_mmu_has_indirect_shadow_pages(vcpu->kvm))
return;
pgprintk("%s: gpa %llx bytes %d\n", __func__, gpa, bytes);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index abfba3cae0ba..22c226f5f4f8 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8588,13 +8588,7 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
/* The instructions are well-emulated on direct mmu. */
if (vcpu->arch.mmu->root_role.direct) {
- unsigned int indirect_shadow_pages;
-
- write_lock(&vcpu->kvm->mmu_lock);
- indirect_shadow_pages = vcpu->kvm->arch.indirect_shadow_pages;
- write_unlock(&vcpu->kvm->mmu_lock);
-
- if (indirect_shadow_pages)
+ if (kvm_mmu_has_indirect_shadow_pages(vcpu->kvm))
kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
return true;
base-commit: 69b4e5b82fec7195c79c939ce25789b16a133f3a
--
