Re: [PATCH v5 19/20] KVM: x86: selftests: Add APX state and ABI test

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The inline assembly in `read_egpr()` lacks the `volatile` qualifier, allowing the compiler to optimize away or cache the EGPR reads.
--

--- Patch [19]: [PATCH v5 19/20] KVM: x86: selftests: Add APX state and ABI test ---
commit 7c0f33a315737d86b103b402a97744d82eb69544
Author: Chang S. Bae <chang.seok.bae@intel.com>

KVM: x86: selftests: Add APX state and ABI test

This commit adds a selftest to verify APX-specific behavior and ABI
interactions for EGPR state. It ensures that the state is saved correctly on
VM entry, retained when APX is disabled, and properly exposed via the XSAVE
ABI.

> diff --git a/tools/testing/selftests/kvm/include/x86/processor.h b/tools/testing/selftests/kvm/include/x86/processor.h
> index 7d3a27bc0d842..5f5acc33486ab 100644
> --- a/tools/testing/selftests/kvm/include/x86/processor.h
> +++ b/tools/testing/selftests/kvm/include/x86/processor.h
[ ... ]
> @@ -877,6 +879,124 @@ static inline void write_sse_reg(int reg, const sse128_t *data)
>  	}
>  }
>  
> +static inline unsigned long read_egpr(int reg)
> +{
> +	unsigned long data = 0;
> +
> +	/* mov %r16..%r31, %rax */
> +	switch (reg) {
> +	case 16:
> +		asm(".byte 0xd5, 0x48, 0x89, 0xc0" : "=a"(data));

[Severity: Medium]
Since this inline assembly has an output operand but lacks the volatile
qualifier, can the compiler optimize away or cache the EGPR reads?

When compiled with optimizations, GCC might treat this as a pure function and
reuse the initially cached values across GUEST_SYNC barriers. If userspace
updates the EGPRs in test_userspace_update(), will the guest compare against
stale cached values instead of emitting instructions to re-read the hardware
registers, causing the test to spuriously fail?

> +		break;
> +	case 17:
> +		asm(".byte 0xd5, 0x48, 0x89, 0xc8" : "=a"(data));
> +		break;
[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260720171949.498680-1-chang.seok.bae@intel.com?part=19