KVM: x86/mmu: Move shadow-present check out of spte_has_volatile_bits()

Move the is_shadow_present_pte() check out of spte_has_volatile_bits()
and into its callers.  Well, caller, since only one of its two callers
doesn't already do the shadow-present check.

Opportunistically move the helper to spte.c/h so that it can be used by
the TDP MMU, which is also the primary motivation for the shadow-present
change.  Unlike the legacy MMU, the TDP MMU uses a single path for clear
leaf and non-leaf SPTEs, and to avoid unnecessary atomic updates, the TDP
MMU will need to check is_last_spte() prior to calling
spte_has_volatile_bits(), and calling is_last_spte() without first
calling is_shadow_present_spte() is at best odd, and at worst a violation
of KVM's loosely defines SPTE rules.

Note, mmu_spte_clear_track_bits() could likely skip the write entirely
for SPTEs that are not shadow-present.  Leave that cleanup for a future
patch to avoid introducing a functional change, and because the
shadow-present check can likely be moved further up the stack, e.g.
drop_large_spte() appears to be the only path that doesn't already
explicitly check for a shadow-present SPTE.

No functional change intended.

Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220423034752.1161007-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 4739b53c9734..e5c0b6db6f2c 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -90,6 +90,34 @@ static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
 				     E820_TYPE_RAM);
 }
 
+/*
+ * Returns true if the SPTE has bits that may be set without holding mmu_lock.
+ * The caller is responsible for checking if the SPTE is shadow-present, and
+ * for determining whether or not the caller cares about non-leaf SPTEs.
+ */
+bool spte_has_volatile_bits(u64 spte)
+{
+	/*
+	 * Always atomically update spte if it can be updated
+	 * out of mmu-lock, it can ensure dirty bit is not lost,
+	 * also, it can help us to get a stable is_writable_pte()
+	 * to ensure tlb flush is not missed.
+	 */
+	if (!is_writable_pte(spte) && is_mmu_writable_spte(spte))
+		return true;
+
+	if (is_access_track_spte(spte))
+		return true;
+
+	if (spte_ad_enabled(spte)) {
+		if (!(spte & shadow_accessed_mask) ||
+		    (is_writable_pte(spte) && !(spte & shadow_dirty_mask)))
+			return true;
+	}
+
+	return false;
+}
+
 bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	       const struct kvm_memory_slot *slot,
 	       unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,