Re: [PATCH] selftests: kvm: Fix an unexpected failure with newer gcc compiler

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/08/20 18:42, Sean Christopherson wrote:
> On Fri, Aug 14, 2020 at 09:21:05PM +0800, Yang Weijiang wrote:
>> If debug_regs.c is built with newer gcc, e.g., 8.3.1 on my side, then the generated
>> binary looks like over-optimized by gcc:
>>
>> asm volatile("ss_start: "
>>              "xor %%rax,%%rax\n\t"
>>              "cpuid\n\t"
>>              "movl $0x1a0,%%ecx\n\t"
>>              "rdmsr\n\t"
>>              : : : "rax", "ecx");
>>
>> is translated to :
>>
>>   000000000040194e <ss_start>:
>>   40194e:       31 c0                   xor    %eax,%eax     <----- rax->eax?
>>   401950:       0f a2                   cpuid
>>   401952:       b9 a0 01 00 00          mov    $0x1a0,%ecx
>>   401957:       0f 32                   rdmsr
>>
>> As you can see rax is replaced with eax in taret binary code.
> 
> It's an optimization.  `xor rax, rax` and `xor eax, eax` yield the exact
> same result, as writing the lower 32 bits of a GPR in 64-bit mode clears
> the upper 32 bits.  Using the eax variant avoids the REX prefix and saves
> a byte of code.

I would have expected that from binutils though, not GCC.

> Use `xor %%eax, %%eax`.  That should always generate a 2 byte instruction.
> Encoding a 64-bit operation would technically be legal, but I doubt any
> compiler would do that in practice.

Indeed, and in addition the clobbers are incorrect since they miss rbx
and rdx.  I've sent a patch.

Paolo