Don't force a TLB if mmu_spte_update() clears Accessed bit, as access
tracking tolerates false negatives, as evidenced by the mmu_notifier hooks
that explicit test and age SPTEs without doing a TLB flush.
In practice, this is very nearly a nop. spte_write_protect() and
spte_clear_dirty() never clear the Accessed bit. make_spte() always
sets the Accessed bit for !prefetch scenarios. FNAME(sync_spte) only sets
SPTE if the protection bits are changing, i.e. if a flush will be needed
regardless of the Accessed bits. And FNAME(pte_prefetch) sets SPTE if and
only if the old SPTE is !PRESENT.
That leaves kvm_arch_async_page_ready() as the one path that will generate
a !ACCESSED SPTE *and* overwrite a PRESENT SPTE. And that's very arguably
a bug, as clobbering a valid SPTE in that case is nonsensical.
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
arch/x86/kvm/mmu/mmu.c | 31 +++++++++----------------------
1 file changed, 9 insertions(+), 22 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 0a6c6619d213..77d1072b130d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -515,37 +515,24 @@ static u64 mmu_spte_update_no_track(u64 *sptep, u64 new_spte)
* TLBs must be flushed. Otherwise rmap_write_protect will find a read-only
* spte, even though the writable spte might be cached on a CPU's TLB.
*
+ * Remote TLBs also need to be flushed if the Dirty bit is cleared, as false
+ * negatives are not acceptable, e.g. if KVM is using D-bit based PML on VMX.
+ *
+ * Don't flush if the Accessed bit is cleared, as access tracking tolerates
+ * false negatives, and the one path that does care about TLB flushes,
+ * kvm_mmu_notifier_clear_flush_young(), uses mmu_spte_update_no_track().
+ *
* Returns true if the TLB needs to be flushed
*/
static bool mmu_spte_update(u64 *sptep, u64 new_spte)
{
- bool flush = false;
u64 old_spte = mmu_spte_update_no_track(sptep, new_spte);
if (!is_shadow_present_pte(old_spte))
return false;
- /*
- * For the spte updated out of mmu-lock is safe, since
- * we always atomically update it, see the comments in
- * spte_has_volatile_bits().
- */
- if (is_mmu_writable_spte(old_spte) &&
- !is_writable_pte(new_spte))
- flush = true;
-
- /*
- * Flush TLB when accessed/dirty states are changed in the page tables,
- * to guarantee consistency between TLB and page tables.
- */
-
- if (is_accessed_spte(old_spte) && !is_accessed_spte(new_spte))
- flush = true;
-
- if (is_dirty_spte(old_spte) && !is_dirty_spte(new_spte))
- flush = true;
-
- return flush;
+ return (is_mmu_writable_spte(old_spte) && !is_writable_pte(new_spte)) ||
+ (is_dirty_spte(old_spte) && !is_dirty_spte(new_spte));
}
/*
--
2.44.0.291.gc1ea87d7ee-goog
