[PATCH v3 2/4] KVM: TDX: Set bits 31:16 to 0 for the synthesized Exit Reason

[Xiaoyao Li]

Set bits 31:16 to 0 instead of all-1s for KVM's synthesized Exit Reason.

KVM is going to support Bus Lock VM exit for TDX, after which bit 26 of
the Exit Reason becomes meaningful and indicates that a bus lock happened.
The existing synthesized Exit Reason, -1u, will cause a false positive in
that case.  Change the synthesized Exit Reason from -1u to U16_MAX, so that
bits 31:16 are set to 0.  This also avoids the potential issues when other
bits in 31:16 become valid in the future.

As a bonus, the check for synthesized Exit Reason in tdx_failed_vmentry()
becomes unnecessary. Drop it.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
Note, the checking of (*reason != TDX_INVALID_EXIT_REASON) in
tdx_get_exit_info() can cause a false-positive when the real exit reason
is EPT_MISCONFIG. This issue is orthogonal to enabling Bus Lock VM exit and
it's not urgent since EPT_MISCONFIIG is not supposed to happen unless
current KVM code is buggy. We leave the fix for this issue to the future.

Note, #2, the checking of tdx_failed_vmentry() seems to miss the case
where a real EPT_MISCONFIG happens with failed_vmentry being set.
First, in practice, EPT_MISCONFIG cannot happen with failed_vmentry being
set. Second, even if it can, this is an pre-existing issue and the next
patch can address it.

Changes in v3:
- split from the patch 2 in v2.
- define a MARCO for the synthesized invalid Exit Reason.
---
 arch/x86/kvm/vmx/tdx.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 7338ac0af693..df23db9430f0 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -921,6 +921,9 @@ static __always_inline u32 tdcall_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 	return EXIT_REASON_TDCALL;
 }
 
+/* Synthesized invalid Exit Reason */
+#define TDX_INVALID_EXIT_REASON		U16_MAX
+
 static __always_inline u32 tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
@@ -934,7 +937,12 @@ static __always_inline u32 tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 	case TDX_NON_RECOVERABLE_TD_WRONG_APIC_MODE:
 		break;
 	default:
-		return -1u;
+		/*
+		 * Return the synthesized invalid Exit Reason, as the TDX
+		 * module never attempted to run the vCPU, i.e. the Exit
+		 * Reason is undefined, but this is NOT a failed VM-Enter
+		 */
+		return TDX_INVALID_EXIT_REASON;
 	}
 
 	exit_reason = tdx->vp_enter_ret;
@@ -950,7 +958,7 @@ static __always_inline u32 tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 		 * Defer KVM_BUG_ON() until tdx_handle_exit() because this is in
 		 * non-instrumentable code with interrupts disabled.
 		 */
-		return -1u;
+		return TDX_INVALID_EXIT_REASON;
 	default:
 		break;
 	}
@@ -981,8 +989,7 @@ static noinstr void tdx_vcpu_enter_exit(struct kvm_vcpu *vcpu)
 
 static bool tdx_failed_vmentry(struct kvm_vcpu *vcpu)
 {
-	return vmx_get_exit_reason(vcpu).failed_vmentry &&
-	       vmx_get_exit_reason(vcpu).full != -1u;
+	return vmx_get_exit_reason(vcpu).failed_vmentry;
 }
 
 static fastpath_t tdx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
@@ -2144,7 +2151,7 @@ void tdx_get_exit_info(struct kvm_vcpu *vcpu, u32 *reason,
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
 
 	*reason = tdx->vt.exit_reason.full;
-	if (*reason != -1u) {
+	if (*reason != TDX_INVALID_EXIT_REASON) {
 		*info1 = vmx_get_exit_qual(vcpu);
 		*info2 = tdx->ext_exit_qualification;
 		*intr_info = vmx_get_intr_info(vcpu);
-- 
2.43.0