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

Gleb Natapov
2012-12-22T16:20:52Z

On Sat, Dec 22, 2012 at 05:51:08PM +0200, Avi Kivity wrote:
> 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.
True :(. Actually if we will move from OP_NONE to NoWrite we can drop
saved_dst_type hack from x86_emulate_insn.

--
Gleb.