[PATCH 15/18] KVM: x86/mmu: Dedup logic for detecting TLB flushes on leaf SPTE changes

Sean Christopherson
2024-10-11T02:11:25Z

Now that the shadow MMU and TDP MMU have identical logic for detecting
required TLB flushes when updating SPTEs, move said logic to a helper so
that the TDP MMU code can benefit from the comments that are currently
exclusive to the shadow MMU.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/mmu.c | 19 +------------------
arch/x86/kvm/mmu/spte.h | 29 +++++++++++++++++++++++++++++
arch/x86/kvm/mmu/tdp_mmu.c | 3 +--
3 files changed, 31 insertions(+), 20 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 5be3b5f054f1..f75915ff33be 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -488,23 +488,6 @@ static void mmu_spte_set(u64 *sptep, u64 new_spte)
/* Rules for using mmu_spte_update:
* Update the state bits, it means the mapped pfn is not changed.
*
- * If the MMU-writable flag is cleared, i.e. the SPTE is write-protected for
- * write-tracking, remote TLBs must be flushed, even if the SPTE was read-only,
- * as KVM allows stale Writable TLB entries to exist. When dirty logging, KVM
- * flushes TLBs based on whether or not dirty bitmap/ring entries were reaped,
- * not whether or not SPTEs were modified, i.e. only the write-tracking case
- * needs to flush at the time the SPTEs is modified, before dropping mmu_lock.
- *
- * 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(), flushes if a young SPTE is found, i.e.
- * doesn't rely on lower helpers to detect the need to flush.
- *
- * Lastly, don't flush if the Dirty bit is cleared, as KVM unconditionally
- * flushes when enabling dirty logging (see kvm_mmu_slot_apply_flags()), and
- * when clearing dirty logs, KVM flushes based on whether or not dirty entries
- * were reaped from the bitmap/ring, not whether or not dirty SPTEs were found.
- *
* Returns true if the TLB needs to be flushed
*/
static bool mmu_spte_update(u64 *sptep, u64 new_spte)
@@ -527,7 +510,7 @@ static bool mmu_spte_update(u64 *sptep, u64 new_spte)
WARN_ON_ONCE(!is_shadow_present_pte(old_spte) ||
spte_to_pfn(old_spte) != spte_to_pfn(new_spte));

- return is_mmu_writable_spte(old_spte) && !is_mmu_writable_spte(new_spte);
+ return is_tlb_flush_required_for_leaf_spte(old_spte, new_spte);
}

/*
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index c8dc75337c8b..a404279ba731 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -467,6 +467,35 @@ static inline bool is_mmu_writable_spte(u64 spte)
return spte & shadow_mmu_writable_mask;
}

+/*
+ * If the MMU-writable flag is cleared, i.e. the SPTE is write-protected for
+ * write-tracking, remote TLBs must be flushed, even if the SPTE was read-only,
+ * as KVM allows stale Writable TLB entries to exist. When dirty logging, KVM
+ * flushes TLBs based on whether or not dirty bitmap/ring entries were reaped,
+ * not whether or not SPTEs were modified, i.e. only the write-tracking case
+ * needs to flush at the time the SPTEs is modified, before dropping mmu_lock.
+ *
+ * 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(), flushes if a young SPTE is found, i.e.
+ * doesn't rely on lower helpers to detect the need to flush.
+ *
+ * Lastly, don't flush if the Dirty bit is cleared, as KVM unconditionally
+ * flushes when enabling dirty logging (see kvm_mmu_slot_apply_flags()), and
+ * when clearing dirty logs, KVM flushes based on whether or not dirty entries
+ * were reaped from the bitmap/ring, not whether or not dirty SPTEs were found.
+ *
+ * Note, this logic only applies to shadow-present leaf SPTEs. The caller is
+ * responsible for checking that the old SPTE is shadow-present, and is also
+ * responsible for determining whether or not a TLB flush is required when
+ * modifying a shadow-present non-leaf SPTE.
+ */
+static inline bool is_tlb_flush_required_for_leaf_spte(u64 old_spte,
+ u64 new_spte)
+{
+ return is_mmu_writable_spte(old_spte) && !is_mmu_writable_spte(new_spte);
+}
+
static inline u64 get_mmio_spte_generation(u64 spte)
{
u64 gen;
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index f412bca206c5..615c6a84fd60 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1034,8 +1034,7 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
return RET_PF_RETRY;
else if (is_shadow_present_pte(iter->old_spte) &&
(!is_last_spte(iter->old_spte, iter->level) ||
- WARN_ON_ONCE(is_mmu_writable_spte(iter->old_spte) &&
- !is_mmu_writable_spte(new_spte))))
+ WARN_ON_ONCE(is_tlb_flush_required_for_leaf_spte(iter->old_spte, new_spte))))
kvm_flush_remote_tlbs_gfn(vcpu->kvm, iter->gfn, iter->level);

/*
--
2.47.0.rc1.288.g06298d1525-goog