Re: [PATCH v2 2/3] KVM: TDX: Fix the exit reason handling

On 8/10/2026 7:39 PM, [email protected] wrote:
>> @@ -2147,7 +2156,7 @@ 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 (tdx->vt.exit_reason.basic != -1) {
> [Severity: Medium]
> Will this check always evaluate to true due to C integer promotion rules?
> 
> The basic field in union vmx_exit_reason is a 16-bit unsigned bitfield. When
> comparing it to -1, the unsigned 16-bit value is promoted to a signed 32-bit
> integer. If the value was set to -1 (65535), the comparison evaluates as
> 65535 != -1, which is always true.
> 

Well, how about something below on top of this patch?

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index a89885d550c9..e3c8c92f6f71 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -59,6 +59,8 @@ module_param_named(tdx, enable_tdx, bool, 0444);
  #define TDX_SHARED_BIT_PWL_5 gpa_to_gfn(BIT_ULL(51))
  #define TDX_SHARED_BIT_PWL_4 gpa_to_gfn(BIT_ULL(47))

+#define TDX_INVALID_EXIT_REASON                U16_MAX
+
  static const struct tdx_sys_info *tdx_sysinfo;

  void tdh_vp_rd_failed(struct vcpu_tdx *tdx, char *uclass, u32 field, 
u64 err)
@@ -940,7 +942,7 @@ static __always_inline union vmx_exit_reason 
tdx_to_vmx_exit_reason(struct kvm_v
                  * undefined, but this is NOT a failed VM-Enter.
                  */
                 return (union vmx_exit_reason) {
-                       .basic = -1,
+                       .basic = TDX_INVALID_EXIT_REASON,
                 };
         }

@@ -959,7 +961,7 @@ static __always_inline union vmx_exit_reason 
tdx_to_vmx_exit_reason(struct kvm_v
                  * non-instrumentable code with interrupts disabled.
                  */
                 return (union vmx_exit_reason) {
-                       .basic = -1,
+                       .basic = TDX_INVALID_EXIT_REASON,
                 };
         default:
                 break;
@@ -2153,7 +2155,7 @@ 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 (tdx->vt.exit_reason.basic != -1) {
+       if (tdx->vt.exit_reason.basic != TDX_INVALID_EXIT_REASON) {
                 *info1 = vmx_get_exit_qual(vcpu);
                 *info2 = tdx->ext_exit_qualification;
                 *intr_info = vmx_get_intr_info(vcpu);