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

[sashiko-bot@kernel.org]

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?

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

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Sashiko AI review · https://sashiko.dev/#/patchset/20261001-tdx-selftests-v15-0-7c62a5d8a992@google.com?part=20