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

Avi Kivity
2012-12-22T15:51:09Z

On Sat, Dec 22, 2012 at 5:45 PM, Avi Kivity <avi.kivity@gmail.com> wrote:
> 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.

Actually that doesn't work. x86_emulate_insn() needs dst.type in order
to fetch the operand.