Re: [PATCH RESEND] x86: allow VEX et al encodings in 16-bit (protected) mode

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Tue, Jun 25, 2019 at 8:08 AM Jan Beulich <JBeulich@suse.com> wrote:
>
> >>> On 25.06.19 at 16:52, <hjl.tools@gmail.com> wrote:
> > On Tue, Jun 25, 2019 at 12:20 AM Jan Beulich <JBeulich@suse.com> wrote:
> >>
> >> >>> On 24.06.19 at 18:30, <hjl.tools@gmail.com> wrote:
> >> > On Mon, Jun 24, 2019 at 7:05 AM Jan Beulich <JBeulich@suse.com> wrote:
> >> >> --- a/gas/config/tc-i386.c
> >> >> +++ b/gas/config/tc-i386.c
> >> >> @@ -4387,9 +4387,9 @@ md_assemble (char *line)
> >> >>
> >> >>    if (is_any_vex_encoding (&i.tm))
> >> >>      {
> >> >> -      if (flag_code == CODE_16BIT)
> >> >> +      if (!cpu_arch_flags.bitfield.cpui286)
> >> >>         {
> >> >> -         as_bad (_("instruction `%s' isn't supported in 16-bit mode."),
> >> >> +         as_bad (_("instruction `%s' isn't supported outside of protected
> > mode."),
> >> >>                   i.tm.name);
> >> >>           return;
> >> >>         }
> >> >
> >> > flag_code == CODE_16BIT is used to indicate 16-bit mode, in most cases, for
> > real
> >> > mode.   If there are real use cases for VEX/EVEX insns in 16-bit protected
> > mode,
> >> > should we add a directive or pseudo prefix to indicate protected mode
> > encoding?
> >>
> >> Not sure - I'd leave that to you. All I want/need is to be able encode
> >> VEX/XOP/EVEX insns for 16-bit protected mode use. How about we go
> >> with this patch for now, and you further refine things to your liking
> >> subsequently?
> >>
> >
> > Up to now, flag_code == CODE_16BIT disallows VEX/EVEX encoding.
>
> And wrongly so, imo. Hence this patch.
>
> >  I
> > think VEX/EVEX encoding should be allowed only under a new directive.
>
> To be honest, I'm not really up to doing this work, not the least because
> I assume there might be other things that then ought to be made
> protected mode only (and I also wouldn't want to contribute a half baked
> implementation). Hence once again my suggestion: Let's go with this
> patch for now, and if you think things need to be restricted again a little
> more, you can follow up with a change implementing the new directive
> (and all its ramifications beyond VEX/EVEX handling, if any).
>

Is there a real use case to use VEX/EVEX in 16-bit mode?

-- 
H.J.