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

[Jan Beulich]

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.

Jan