Re: [PATCH 2/6] x86/MPX: fix address size handling

["Jan Beulich" <JBeulich at suse dot com>]

>>> On 08.10.13 at 17:32, "H.J. Lu" <hjl.tools@gmail.com> wrote:
> On Tue, Oct 8, 2013 at 8:20 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>> On 08.10.13 at 17:15, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>>> On Tue, Oct 8, 2013 at 7:41 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> While address overrides are ignored in 64-bit mode (and hence shouldn't
>>>> result in an error), trying to use 16-bit addressing is documented to
>>>> result in #UD, and hence the assembler should reject the attempt.
>>>>
>>>> gas/
>>>> 2013-10-08  Jan Beulich <jbeulich@suse.com>
>>>>
>>>>         * tc-i386.c (md_assemble): Alter address size checking for MPX
>>>>         instructions.
>>>>
>>>> --- 2013-10-07/gas/config/tc-i386.c
>>>> +++ 2013-10-07/gas/config/tc-i386.c
>>>> @@ -3549,10 +3549,15 @@ md_assemble (char *line)
>>>>    if (i.bnd_prefix && !i.tm.opcode_modifier.bndprefixok)
>>>>      as_bad (_("expecting valid branch instruction after `bnd'"));
>>>>
>>>> -  if (i.tm.cpu_flags.bitfield.cpumpx
>>>> -      && flag_code == CODE_64BIT
>>>> -      && i.prefix[ADDR_PREFIX])
>>>> -    as_bad (_("32-bit address isn't allowed in 64-bit MPX instructions."));
>>>
>>> It is done on purpose.  When 32-bit address prefix in 64-bit is ignored,
>>> MPX doesn't work correctly for x32.
>>
>> I don't understand: It _is_ being ignored by the hardware as per
>> the documentation. So x32 need to get along with that. Maybe
>> an example would help, so I could understand why you think
>> this _needs_ to be an error...
>>
> 
> X32 won't work with MPX since hardware assumes pointer
> size is always 64 bit in 64-bit mode with or without address
> size prefix.  MPX depends on correct pointer size to work.
> I don't want people to use MPX in x32 by accident.

This seems even more odd - why would x32 be excluded from
using MPX? Again - an example might help, as my understanding
so far was that the implicit zero extension of results of 32-bit
operations should guarantee the half width pointers to be quite
fine to use as full width values (i.e. in other memory operands
I don't see why you would want to use 32-bit addressing either,
except when the wraparound case matters, as might e.g. be
the case with EIP-relative addressing).

And in any case - the assembler shouldn't enforce policy, it
should only enforce architectural restrictions.

Bottom line - I continue to be convinced that the diagnostic
we talk about here ought to be a warning, not an error (at
least by default).

Jan