[PATCH] x86: Allow 16-bit register source for LAR and LSL

[H.J. Lu]

On Mon, Dec 5, 2022 at 3:11 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 03.12.2022 05:13, H.J. Lu wrote:
> > Since LAR and LSL only access 16 bits of the source operand, regardless
> > of operand size, allow 16-bit register source for LAR and LSL, and always
> > disassemble LAR and LSL with 16-bit source operand.
> >
> > gas/
> >
> >       PR gas/29844
> >       * testsuite/gas/i386/i386.s: Add tests for LAR and LSL.
> >       * testsuite/gas/i386/x86_64.s: Likewise.
> >       * testsuite/gas/i386/intelbad.s: Remove "lar/lsl eax, ax".
> >       * testsuite/gas/i386/i386-intel.d: Updated.
> >       * testsuite/gas/i386/i386.d: Likewise.
> >       * testsuite/gas/i386/intel-intel.d: Likewise.
> >       * testsuite/gas/i386/intel.d: Likewise.
> >       * testsuite/gas/i386/intelbad.l: Likewise.
> >       * testsuite/gas/i386/x86_64-intel.d: Likewise.
> >       * testsuite/gas/i386/x86_64.d: Likewise.
> >
> > opcodes/
> >
> >       PR gas/29844
> >       * i386-dis.c (MOD_0F02): Removed.
> >       (MOD_0F03): Likewise.
> >       (dis386_twobyte): Restore larS and lslS.
> >       (mod_table): Remove MOD_0F02 and MOD_0F03.
> >       * i386-opc.tbl: Allow 16-bit register source for LAR and LSL.
> >       * i386-tbl.h: Regenerated.
>
> Please can you refrain from immediately committing patches which have
> a risk of being controversial.
>
> In the case here, given there are uses of the 16-bit register operand
> form in the Linux kernel, I can accept the assembler part of the change.
> The lines in i386-opc.tbl, however, need a comment then, as allowing for
> 16-bit registers despite a wider destination is explicitly not in line
> with the SDM. (Interestingly AMD's PM is different in this regard.)
>
> For the disassembler part you're completely undoing what I did, which is
> wrong - again with reference to the SDM. If you want to accommodate for
> AMD's PM, then you need to vary disassembly according to command line
> options specified, with the default being in line with the SDM (I can
> dig out a pretty old version of the doc, but I believe it has always
> been that way, i.e. even before AMD introduced their clones).
>
> I will revert this change unless you come forward with an adjustment
> within the next couple of days.
>

Given that the only lower 16 bits are used, the 16-bit register source
is more appropriate.  I will raise the issue with the Intel SDM author.

-- 
H.J.