On Mon, Aug 17, 2020 at 01:20:34PM -0400, Paolo Bonzini wrote:

From: Yang Weijiang weijiang.yang@intel.com

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

---

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");
  
  

Hi, Paolo, Should we also change the below expected instruction length(xor) to 2 in accordance with above change?

int ss_size[4] = { 3, /* xor */ 2, /* cpuid */ 5, /* mov */ 2, /* rdmsr */

/* DR6.BD test */ asm volatile("bd_start: mov %%dr0, %%rax" : : : "rax"); -- 2.26.2