Re: [PATCH 3/7] KVM: x86 emulator: introduce NoWrite flag

Avi Kivity
2012-12-22T15:45:34Z

On Sat, Dec 22, 2012 at 5:39 PM, Gleb Natapov <gleb@redhat.com> wrote:
> On Sat, Dec 22, 2012 at 05:28:00PM +0200, Avi Kivity wrote:
>> On Sat, Dec 22, 2012 at 5:11 PM, Gleb Natapov <gleb@redhat.com> wrote:
>> >> #define X2(x...) x, x
>> >> #define X3(x...) X2(x), x
>> >> @@ -1584,6 +1585,9 @@ static int writeback(struct x86_emulate_ctxt *ctxt)
>> >> {
>> >> int rc;
>> >>
>> >> + if (ctxt->d & NoWrite)
>> >> + return X86EMUL_CONTINUE;
>> >> +
>> > Why not make it Dst flag and set dst.type to OP_NONE during decoding
>> > instead of this special case?
>>
>> My thinking was to eventually convert everything to NoWrite, then it's
>> not a special case. But your approach works too, I can change it if
>> you like. We'll have an if () either way though.
> But meanwhile we will have two different ways to disable writeback. When
> all instructions will be marked with NoWrite we can switch it easily if
> we want. Also some instruction can disable writeback conditionally. cmov is one
> (and may be only) of them.

Makes sense, will update for the next iteration.