Re: [PATCH] selftests: kvm: Use a shorter encoding to clear RAX

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, Aug 17, 2020 at 01:20:34PM -0400, Paolo Bonzini wrote:
> From: Yang Weijiang <weijiang.yang@intel.com>

This shouldn't be here without Weijiang's SOB.

> If debug_regs.c is built with newer binutils, the resulting binary is "optimized"
> by the assembler:
> 
> 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 target binary code.
> This causes a difference is the length of xor instruction (2 Byte vs 3 Byte),
> and makes the hard-coded instruction length check fail:
> 
>         /* Instruction lengths starting at ss_start */
>         int ss_size[4] = {
>                 3,              /* xor */   <-------- 2 or 3?
>                 2,              /* cpuid */
>                 5,              /* mov */
>                 2,              /* rdmsr */
>         };
> 
> Encode the shorter version directly and, while at it, fix the "clobbers"
> of the asm.
> 
> Reported-by: Yang Weijiang <weijiang.yang@intel.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>

> ---
>  tools/testing/selftests/kvm/x86_64/debug_regs.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/x86_64/debug_regs.c b/tools/testing/selftests/kvm/x86_64/debug_regs.c
> index 8162c58a1234..b8d14f9db5f9 100644
> --- a/tools/testing/selftests/kvm/x86_64/debug_regs.c
> +++ b/tools/testing/selftests/kvm/x86_64/debug_regs.c
> @@ -40,11 +40,11 @@ static void guest_code(void)
>  
>  	/* Single step test, covers 2 basic instructions and 2 emulated */
>  	asm volatile("ss_start: "
> -		     "xor %%rax,%%rax\n\t"
> +		     "xor %%eax,%%eax\n\t"
>  		     "cpuid\n\t"
>  		     "movl $0x1a0,%%ecx\n\t"
>  		     "rdmsr\n\t"
> -		     : : : "rax", "ecx");
> +		     : : : "eax", "ebx", "ecx", "edx");
>  
>  	/* DR6.BD test */
>  	asm volatile("bd_start: mov %%dr0, %%rax" : : : "rax");
> -- 
> 2.26.2
>