[PATCH v3 3/4] KVM: TDX: Don't assume exit_reason[31:16] as all-0 in tdx_to_vmx_exit_reason()

When handling the real Exit Reason, don't assume the upper 16 bits as
all-0 in tdx_to_vmx_exit_reason(), in preparation for enabling Bus Lock
VM exit.

When Bus Lock VM exit is enabled, the bit 26 of Exit Reason becomes
valid and it can be 1 with various exit reasons. Change the logic in
tdx_to_vmx_exit_reason() to check the basic Exit Reason for correctness.
Also preserve the bit 31:16 when changing the (basic) Exit Reason, to
not lose the information in bit[31:16].

Change the return type of tdx_to_vmx_exit_reason() to
"union vmx_exit_reason" for the convenience of manipulating the basic
field.

Cc: [email protected]
Signed-off-by: Xiaoyao Li <[email protected]>
---
Changes in v3:
- new patch split from patch 2 of v2.
---
 arch/x86/kvm/vmx/tdx.c | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index df23db9430f0..598b85d772e3 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -924,10 +924,10 @@ static __always_inline u32 tdcall_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 /* Synthesized invalid Exit Reason */
 #define TDX_INVALID_EXIT_REASON		U16_MAX
 
-static __always_inline u32 tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
+static __always_inline union vmx_exit_reason tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
-	u32 exit_reason;
+	union vmx_exit_reason exit_reason;
 
 	switch (tdx->vp_enter_ret & TDX_SEAMCALL_STATUS_MASK) {
 	case TDX_SUCCESS:
@@ -942,23 +942,27 @@ static __always_inline u32 tdx_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
 		 * 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;
+		return (union vmx_exit_reason) {
+			.basic = TDX_INVALID_EXIT_REASON,
+		};
 	}
 
-	exit_reason = tdx->vp_enter_ret;
+	exit_reason.full = (u32)tdx->vp_enter_ret;
 
-	switch (exit_reason) {
+	switch (exit_reason.basic) {
 	case EXIT_REASON_TDCALL:
 		if (tdvmcall_exit_type(vcpu))
-			return EXIT_REASON_VMCALL;
-
-		return tdcall_to_vmx_exit_reason(vcpu);
+			exit_reason.basic = EXIT_REASON_VMCALL;
+		else
+			exit_reason.basic = tdcall_to_vmx_exit_reason(vcpu);
+		break;
 	case EXIT_REASON_EPT_MISCONFIG:
 		/*
 		 * Defer KVM_BUG_ON() until tdx_handle_exit() because this is in
 		 * non-instrumentable code with interrupts disabled.
 		 */
-		return TDX_INVALID_EXIT_REASON;
+		exit_reason.basic = TDX_INVALID_EXIT_REASON;
+		break;
 	default:
 		break;
 	}
@@ -975,7 +979,7 @@ static noinstr void tdx_vcpu_enter_exit(struct kvm_vcpu *vcpu)
 
 	tdx->vp_enter_ret = tdh_vp_enter(&tdx->vp, &tdx->vp_enter_args);
 
-	vt->exit_reason.full = tdx_to_vmx_exit_reason(vcpu);
+	vt->exit_reason = tdx_to_vmx_exit_reason(vcpu);
 
 	vt->exit_qualification = tdx->vp_enter_args.rcx;
 	tdx->ext_exit_qualification = tdx->vp_enter_args.rdx;
-- 
2.43.0