Re: [PATCH] KVM: VMX: Formally define KVM's "0xdead" exit reason used for failed VM-Enter

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Mon, 2026-09-21 at 16:56 -0700, Sean Christopherson wrote:
> Add EXIT_REASON_UNDEFINED to track KVM's magic "0xdead" value that's used
> to stuff the exit reason on a failed VM-Enter so that the magic can be
> reused by TDX, and to make it more obvious that it's an arbitrary, KVM-
> defined value (though the whole "dead" thing makes it pretty darn obvious).
> 
> Opportunistically add a compile-time assert to ensure the KVM-defined exit
> reason never collides with a "real" exit reason.
> 
> No functional change intended.
> 
> Cc: Xiaoyao Li <xiaoyao.li@intel.com>
> Cc: Yan Zhao <yan.y.zhao@intel.com>
> Cc: Binbin Wu <binbin.wu@linux.intel.com>
> Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>

> Cc: stable@vger.kernel.org

?? To help the backport of the DOS stuff? Seems strange: "no functional change
intended, CC stable"

Otherwise,
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/include/asm/vmx.h | 8 ++++++++
>  arch/x86/kvm/vmx/vmx.c     | 4 +++-
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> index 3f1b3096ff04..e49b836c7313 100644
> --- a/arch/x86/include/asm/vmx.h
> +++ b/arch/x86/include/asm/vmx.h
> @@ -20,6 +20,14 @@
>  #include <asm/trapnr.h>
>  #include <asm/vmxfeatures.h>
>  
> +/*
> + * UNDEFINED is a KVM-defined value used to indicate that the basic exit reason
> + * is undefined/invalid, e.g. on VM-Fail or if TDX never attempted VM-Enter.
> + * The value itself is arbitrary, it just needs to not collide with a real exit
> + * reason.
> + */
> +#define EXIT_REASON_UNDEFINED			0xdead
> +
>  struct vmcs_hdr {
>  	u32 revision_id:31;
>  	u32 shadow_vmcs:1;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 612ab07d4100..c897dbf0e954 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6388,6 +6388,8 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
>  static const int kvm_vmx_max_exit_handlers =
>  	ARRAY_SIZE(kvm_vmx_exit_handlers);
>  
> +static_assert(EXIT_REASON_UNDEFINED >= ARRAY_SIZE(kvm_vmx_exit_handlers));
> +
>  void vmx_get_exit_info(struct kvm_vcpu *vcpu, u32 *reason,
>  		       u64 *info1, u64 *info2, u32 *intr_info, u32 *error_code)
>  {
> @@ -7474,7 +7476,7 @@ static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
>  	vmx_enable_fb_clear(vmx);
>  
>  	if (unlikely(vmx->fail)) {
> -		vmx->vt.exit_reason.full = 0xdead;
> +		vmx->vt.exit_reason.full = EXIT_REASON_UNDEFINED;
>  		goto out;
>  	}
>