Re: [Xen-devel] [PATCH v2 2/3] x86emul: correct handling of FPU insns faulting on memory write

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.03.17 at 14:24, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 03/15/2017 06:28 AM, Jan Beulich wrote:
>> @@ -3716,9 +3720,9 @@ x86_emulate(
>>          break;
>>  
>>      case 0x9b:  /* wait/fwait */
>> -        fic.insn_bytes = 1;
>>          host_and_vcpu_must_have(fpu);
>>          get_fpu(X86EMUL_FPU_wait, &fic);
>> +        fic.insn_bytes = 1;
>>          asm volatile ( "fwait" ::: "memory" );
>>          check_fpu_exn(&fic);
>>          break;
> 
> Why is this needed?

This isn't strictly needed, but desirable, due to the conditional being
added in

@@ -7916,7 +7920,7 @@ x86_emulate(
     ctxt->regs->eflags &= ~X86_EFLAGS_RF;
 
  done:
-    put_fpu(&fic, ctxt, ops);
+    put_fpu(&fic, fic.insn_bytes > 0 && dst.type == OP_MEM, ctxt, ops);
     put_stub(stub);
     return rc;
 #undef state

(both host_and_vcpu_must_have() and get_fpu() may end up
branching to "done"). Everywhere else the field is already being
set after such basic checks.

Jan


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