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

On Mon, 2026-08-10 at 19:21 +0800, Xiaoyao Li wrote:
> Get and check the exit reason from the low 16 bits of vp_enter_ret, and
> store the synthesized/transformed exit reason in the "basic" field.
> 
> Some bits in the upper 16 bits in the exit reason have their own meanings
> and they might be 1.  When handling the exit reason, only do handling on
> the lower 16 bits and keep the upper 16 bits unchanged.  This change
> also helps remove the additional check in tdx_failed_vmentry().
> 
> Note, due to the synthesized invalid exit reason, -1, is changed to
> assigned to the "basic" field, adjust the checking in tdx_get_exit_info()
> accordingly.
> 
> Fixes: 095b71a03f49 ("KVM: TDX: Add a place holder to handle TDX VM exit")
> Fixes: c42856af8f70 ("KVM: TDX: Add a place holder for handler of TDX hypercalls (TDG.VP.VMCALL)")
> Cc: [email protected]
> Signed-off-by: Xiaoyao Li <[email protected]>
> ---
> Changes in v2:
>  - new patch
> ---
>  arch/x86/kvm/vmx/tdx.c | 35 ++++++++++++++++++++++-------------
>  1 file changed, 22 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 7338ac0af693..a89885d550c9 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -921,10 +921,10 @@ static __always_inline u32 tdcall_to_vmx_exit_reason(struct kvm_vcpu *vcpu)
>  	return EXIT_REASON_TDCALL;
>  }
>  
> -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:
> @@ -934,23 +934,33 @@ 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;
> +		/*
> +		 * Synthesize an invalid bogus Exit Reason, as the TDX-Module

I think this blurb came from Sean, but can we standardize on "TDX module"? The
code currently uses "TDX module" and "TDX-module" and "TDX-module" used much
less. I also don't see why it needs the "-".

> +		 * never attempted to run the vCPU, i.e. the Exit Reason is
> +		 * undefined, but this is NOT a failed VM-Enter.
> +		 */
> +		return (union vmx_exit_reason) {
> +			.basic = -1,
> +		};
>  	}
>  
> -	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 -1u;
> +		return (union vmx_exit_reason) {
> +			.basic = -1,
> +		};

We could make this return instead be a goto err; that returns this. Bonus is the
comment on the other one can cover them both. What do you think?

>  	default:
>  		break;
>  	}
> @@ -967,7 +977,7 @@ 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;
> @@ -981,8 +991,7 @@ 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 +2153,7 @@ 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) {
>  		*info1 = vmx_get_exit_qual(vcpu);
>  		*info2 = tdx->ext_exit_qualification;
>  		*intr_info = vmx_get_intr_info(vcpu);