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

[Sean Christopherson <seanjc@google.com>]

On Mon, Aug 10, 2026, Sean Christopherson wrote:
> On Mon, Aug 10, 2026, Xiaoyao Li wrote:
> > On 8/10/2026 7:39 PM, sashiko-bot@kernel.org 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) {

This would arguably be a bug fix as well, because the "real" EXIT_REASON_EPT_MISCONFIG
path gets a false negative.  E.g. when getting information for an EPT Misconfig
for the tracepoint, KVM really should print all information, not zeros.

At a glance, this exact change can probably be a separate patch too.

> 		<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/anXxBzO41_5eaaOI@google.com