Re: [Xen-devel] [PATCH v9 1/2] x86emul: New return code for unimplemented instruction

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.09.17 at 17:23, <ppircalabu@xxxxxxxxxxxxxxx> wrote:
> On Lu, 2017-09-04 at 23:42 -0600, Jan Beulich wrote:
>> > >
>> > > >
>> > > > @@ -5177,7 +5177,7 @@ x86_emulate(
>> > > >                  goto done;
>> > > >              break;
>> > > >          default:
>> > > > -            goto cannot_emulate;
>> > > > +            goto unimplemented_insn;
>> > > While I can see why you do this change, for many/all of the ones
>> > > I'll leave in context below I think you rather want to switch to
>> > > generate_exception(EXC_UD).
>> > Some of the opcodes are valid but not supported by the emulator. In
>> > this case X86EMUL_UNIMPLEMENTED should be returned to allow the
>> > monitor
>> > app to handle this case. Also, in the worst case scenario, when the
>> > opcode doesn't correspond to a valid x86(-64) instruction, if the
>> > monitor app for example tries to single-step it on the real
>> > hardware an
>> > UD exception will also be reported.
>> Please be more precise with "some of the opcodes are valid". When I
>> looked through your change, I don't think I've seen any such case for
>> the
>> places I meant the comment to apply to. Also, as far as the emulator
>> changes themselves go, please leave aside considerations of what a
>> monitor app may or may not do. These changes need to be consistent
>> all by themselves.
>>
> Sorry about the poor wording. I was under the impression that you
> required the invalid opcodes to be handled differently from the ones
> which are valid but unimplemented (directly generate EXC_UD instead of
> returning X86EMUL_UNIMPLEMENTED and let the caller handle it).

Yes, that's what I think would be best. I admit there is a potential
problem with this, when one or more of them become defined. But
that's something I'd like to also have Andrew's input on.

> e.g. for  X86EMUL_OPC_VEX(0x0f38, 0xf3): /* Grp 17 */
> the mod R/M possible values are 1,2,3 (source: http://sandpile.org/x86/ 
> opc_grp.htm).
> From my perspective I wouldn't differentiate between those 2 cases and
> return X86EMUL_UNIMPLEMENTED.

I must still be missing something: What two cases are you talking
about? The three valid values have an implementation in the
emulator. All others are undefined, i.e. at present would ideally
produce #UD (see above).

Jan


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