[PATCH] x86: Incorrect vmgexit entry in prefix_table

[Jan Beulich]

On 17.06.2020 16:11, H.J. Lu wrote:
> On Wed, Jun 17, 2020 at 7:10 AM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 17.06.2020 16:02, H.J. Lu wrote:
>>> On Tue, Jun 16, 2020 at 7:11 AM H.J. Lu via Binutils
>>> <binutils@sourceware.org> wrote:
>>>>
>>>> From: "Cui,Lili" <lili.cui@intel.com>
>>>>
>>>>         * i386-dis.c (prefix_table): Delete the incorrect vmgexit.
>>>> ---
>>>>  opcodes/i386-dis.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/opcodes/i386-dis.c b/opcodes/i386-dis.c
>>>> index 441866d6c97..55e595a4be0 100644
>>>> --- a/opcodes/i386-dis.c
>>>> +++ b/opcodes/i386-dis.c
>>>> @@ -3577,7 +3577,7 @@ static const struct dis386 prefix_table[][4] = {
>>>>      { "vmmcall",       { Skip_MODRM }, 0 },
>>>>      { "vmgexit",       { Skip_MODRM }, 0 },
>>>>      { Bad_Opcode },
>>>> -    { "vmgexit",       { Skip_MODRM }, 0 },
>>>> +    { Bad_Opcode },
>>>>    },
>>>>
>>>>    /* PREFIX_0F01_REG_5_MOD_0 */
>>>> --
>>>> 2.26.2
>>>>
>>>
>>> This is the patch I am checking in.
>>
>> But both patches are wrong - as per the PM there are two encodings,
>> i.e. either prefix is valid. Afaics from the quoted text above (I
>> didn't get to see the original mail, presumably due to email issues
>> over here) there's no justification here at all why the second
>> entry would be invalid. Please revert.
> 
> Please add a testcase to verify it.

I didn't at the time I added support for it primarily because
there's no way to sensibly express this to gas. And I'm very
hesitant to add .byte-based tests (except when wanting to test
the disassembler against invalid encodings) ... In any event
the bad commit needs reverting first, or any such test added
would fail. And also in any event it would have been prudent
to check why things are the way they are, before "fixing" them.
At that point one of the two of you could have decided to add
such a test to your liking.

Jan