Re: [PATCH v15 20/23] KVM: selftests: Add a helper to issue TDVMCALLs from the TDX vm

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 10/2/2026 5:13 PM, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] kvm: selftests: __tdvmcall silently masks TDCALL hardware failures
> --
> 
> commit 7709c4e900a0fe6433017aebbf708ee2946d3f19
> Author: Erdem Aktas <erdemaktas@google.com>
> 
> KVM: selftests: Add a helper to issue TDVMCALLs from the TDX vm
> 
> This commit introduces a helper function __tdvmcall() to allow TDX VMs
> to issue TDG.VP.VMCALL with a single function call, abstracting the
> register setup. The function passes the GHCI sub-function and arguments,
> and returns the TDVMCALL status.
> 
>> diff --git a/tools/testing/selftests/kvm/lib/x86/tdx/tdx.S b/tools/testing/selftests/kvm/lib/x86/tdx/tdx.S
>> new file mode 100644
>> index 0000000000000..e3f0f7f2232bf
>> --- /dev/null
>> +++ b/tools/testing/selftests/kvm/lib/x86/tdx/tdx.S
>> @@ -0,0 +1,39 @@
> [ ... ]
>> +	mov $0, %rax		/* TDCALL leaf 0: TDG.VP.VMCALL */
>> +	mov $0xFC00, %rcx	/* Expose R10-R15 */
>> +	mov $0, %r10		/* Standard GHCI call */
>> +
>> +	/* TDCALL instruction */
>> +	.byte 0x66, 0x0f, 0x01, 0xcc
>> +
>> +	/* Extract status */
>> +	mov %r10, %rax
> 
> [Severity: Medium]
> Does this code inadvertently discard the TDCALL hardware failure status?
> 
> The TDCALL instruction returns its module-level execution status in %rax.
> If TDCALL fails (e.g., due to invalid arguments or incorrect VM state), %rax
> contains a non-zero error code and output registers like %r10 are left
> unmodified (which was initialized to 0 prior to the call).
> 
> By unconditionally overwriting %rax with %r10, any TDCALL failure status
> appears to be silently masked and converted into a false success. Should
> this code check or preserve the error code in %rax before overwriting it?

This is a wrong comment from Sashiko. RAX returned from TDG.VP.VMCALL is always 0.

>> +
>> +	pop %r15
>> +	pop %r14
>> +	pop %r13
>> +	pop %r12
>> +	ret
>