[PATCH v4 06/18] KVM: x86: Ensure runtime reads of disabled_quirks are resolved once

[Sean Christopherson <seanjc@google.com>]

Wrap the sole reader of disabled_quirks with READ_ONCE(), and wrap the
post-VM-creation write to disabled_quirks with WRITE_ONCE(), to ensure
checking the status of a quirk doesn't re-read disabled_quirks *if* the
caller needs such a guarantee.  This will allow splitting the "fast" MMU
zap into front and back halves, without potentially skipping the back
half if SLOT_ZAP_ALL were concurrently disabled (which would be "fine" in
the current code base, but far from ideal).

Cc: stable@vger.kernel.org # 6.12.x
Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 3 ++-
 arch/x86/kvm/x86.h | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 226c6cfe8062..8abd733d5173 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3940,7 +3940,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		fallthrough;
 	case KVM_CAP_DISABLE_QUIRKS:
 		mutex_lock(&kvm->lock);
-		kvm->arch.disabled_quirks |= cap->args[0] & kvm_caps.supported_quirks;
+		WRITE_ONCE(kvm->arch.disabled_quirks,
+			   kvm->arch.disabled_quirks | (cap->args[0] & kvm_caps.supported_quirks));
 		mutex_unlock(&kvm->lock);
 		r = 0;
 		break;
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 8ece468087a8..75f13d88db58 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -304,7 +304,7 @@ static inline bool vcpu_match_mmio_gpa(struct kvm_vcpu *vcpu, gpa_t gpa)
 
 static inline bool kvm_check_has_quirk(struct kvm *kvm, u64 quirk)
 {
-	return !(kvm->arch.disabled_quirks & quirk);
+	return !(READ_ONCE(kvm->arch.disabled_quirks) & quirk);
 }
 
 static __always_inline void kvm_request_l1tf_flush_l1d(void)
-- 
2.55.0.795.g602f6c329a-goog