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

On 8/11/2026 7:57 AM, Sean Christopherson wrote:
> On Mon, Aug 10, 2026, Xiaoyao Li wrote:
>> 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?
> 
> No, we need to not rely on magic exit_reason.basic values.  Can't this be?
> 
> 	if ((tdx->vp_enter_ret & TDX_SW_ERROR) != TDX_SW_ERROR) {

I think the reason of checking if (*reason != -1u) in the original 
commit 095b71a03f49 ("KVM: TDX: Add a place holder to handle TDX VM 
exit") was that, -1u means there is no valid Exit Reason in the lower 32 
bits in vp_enter_ret.

Change it to check TDX_SW_ERROR, doesn't look correct to me. Set the 
EPT_MISCONFIG magic handling aside, TDX_SW_ERROR only means the SEAMCALL 
instruction faults, e.g., hitting #UD, #GP, or VMFAILINVALID. Just a 
small subset of the cases where there is no valid Exit Reason.

> 		<read values>
> 	} else {
> 		<zero values>
> 	}
> 
> We'd need to use the unsafe version if we go with my suggestion[*], but other
> than that wrinkle, the above seems like the obviously correct fix (maybe too
> obvious)?
> 
> [*] https://lore.kernel.org/all/[email protected]