• Yang Weijiang's avatar
    selftests: kvm: Use a shorter encoding to clear RAX · 98b0bf02
    Yang Weijiang authored
    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.
    
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarYang Weijiang <weijiang.yang@intel.com>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    98b0bf02
debug_regs.c 6.07 KB