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

["Chang S. Bae" <chang.seok.bae@intel.com>]

On 7/20/2026 11:17 AM, sashiko-bot@kernel.org wrote:
>>   
>> +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?
Without -mapxf, the compiler shouldn't include EGPRs in the optimization 
options I think.

Thanks,
Chang