[PATCH v2 5/9] x86: improve handling of insns with ambiguous operand sizes

Fri Nov 8 08:09:00 GMT 2019

On 07.11.2019 18:47,  H.J. Lu  wrote:
> On Thu, Nov 7, 2019 at 2:27 AM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 06.11.2019 23:54,  H.J. Lu  wrote:
>>> On Mon, Nov 4, 2019 at 11:45 PM Jan Beulich <jbeulich@suse.com> wrote:
>>>>
>>>> On 04.11.2019 18:12, H.J. Lu wrote:
>>>>> On Mon, Nov 4, 2019 at 2:29 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>>>
>>>>>> On 31.10.2019 18:26,  H.J. Lu  wrote:
>>>>>>> On Thu, Oct 31, 2019 at 2:24 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>>>>>
>>>>>>>> On 31.10.2019 00:57, H.J. Lu wrote:
>>>>>>>>> On Wed, Oct 30, 2019 at 12:59 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>>>>>>>
>>>>>>>>>> On 29.10.2019 18:55,  H.J. Lu  wrote:
>>>>>>>>>>> On Mon, Oct 28, 2019 at 1:05 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>> Commit b76bc5d54e ("x86: don't default variable shift count insns to
>>>>>>>>>>>> 8-bit operand size") pointed out a very bad case, but the underlying
>>>>>>>>>>>> problem is, as mentioned on various occasions, much larger: Silently
>>>>>>>>>>>> selecting a (nowhere documented afaict) certain default operand size
>>>>>>>>>>>> when there's no "sizing" suffix and no suitable register operand(s) is
>>>>>>>>>>>> simply dangerous (for the programmer to make mistakes).
>>>>>>>>>>>>
>>>>>>>>>>>> While in Intel syntax mode such mistakes already lead to an error (which
>>>>>>>>>>>> is going to remain that way), AT&T syntax mode now gains warnings in
>>>>>>>>>>>> such cases by default, which can be suppressed or promoted to an error
>>>>>>>>>>>> if so desired by the programmer. Furthermore at least general purpose
>>>>>>>>>>>> insns now consistently have a default applied (alongside the warning
>>>>>>>>>>>> emission), rather than accepting some and refusing others.
>>>>>>>>>>>>
>>>>>>>>>>>> No warnings are (as before) to be generated for "DefaultSize" insns as
>>>>>>>>>>>> well as ones acting on selector and other fixed-width values. The set of
>>>>>>>>>>>> "DefaultSize" ones gets slightly widened for the purposes here.
>>>>>>>>>>>
>>>>>>>>>>> What is the advantage to add DefaultSize vs the alternative?
>>>>>>>>>>
>>>>>>>>>> I don't know what alternative you refer to; if you mean some
>>>>>>>>>> hypothetical one, then the advantage of simply adding
>>>>>>>>>> DefaultSize as done here is likely that it allows to not add or
>>>>>>>>>> further complicate logic in tc-i386*.c. Furthermore the ones which
>>>>>>>>>> get the attribute added should have had it already before, if the
>>>>>>>>>> comment "default insn size depends on mode" is to be trusted.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> DefaultSize is added to some instructions and then they are excluded:
>>>>>>>>>
>>>>>>>>> +          /* exclude jmp/ljmp */
>>>>>>>>> +          && strcmp (i.tm.name, "jmp") && strcmp (i.tm.name, "ljmp")
>>>>>>>>> +          /* exclude byte-displacement jumps */
>>>>>>>>> +          && !i.tm.opcode_modifier.jumpbyte
>>>>>>>>> +          /* exclude lgdt/lidt/sgdt/sidt */
>>>>>>>>> +          && (i.tm.base_opcode != 0x0f01 || i.tm.extension_opcode > 3)
>>>>>>>>>            /* exclude fldenv/frstor/fsave/fstenv */
>>>>>>>>>            && i.tm.opcode_modifier.no_ssuf)
>>>>>>>>>
>>>>>>>>> It looks odd.
>>>>>>>>
>>>>>>>> But this isn't the only place where defaultsize gets evaluated.
>>>>>>>> See how lgdt/lidt/sgdt/sidt already have the attribute in the
>>>>>>>> opcode table, but need exclusion here now too. The alternative
>>>>>>>> would be two independent attributes - one to be evaluated here,
>>>>>>>> and the other to be evaluated further down in the function. Yet
>>>>>>>> again - this dual use has been there before, and just needs
>>>>>>>> suitable extending of the logic now.
>>>>>>>>
>>>>>>>
>>>>>>> Normally instructions with DefaultSize have i.suffix unset.  Except with
>>>>>>> .code16gcc, which is used to support 16-bit mode with 32-bit address,
>>>>>>> i.suffix is set to 'l' for 32-bit address.
>>>>>>
>>>>>> I don't follow you here: Since when is there a connection between
>>>>>> 'l' suffix and addressing mode? All .code16gcc distinguishes from
>>>>>> plain .code16 is stack pointer width, isn't it? In which case
>>>>>> using fldenv etc in their 32-bit operand size form looks wrong.
>>>>>
>>>>> fldenv doesn't use 32-bit operand size.
>>>>
>>>> Then what is it that DefaultSize is needed for on its template?
>>>>
>>>>>> Or is this behavior firmly documented? The main gas documentation
>>>>>> certainly doesn't, afaics.
>>>>>
>>>>> I don't think code16gcc is well documented.
>>>>
>>>> Which is not very helpful.
>>>>
>>>>>>> However, iret/fldenv/frstor/fsave/fstenv
>>>>>>> are exceptions since they need 16-bit variants.  So we need 2 different
>>>>>>> DefaultSize behaviors for .code16gcc, one uses LONG_MNEM_SUFFIX
>>>>>>> and the other uses WORD_MNEM_SUFFIX.  We should update
>>>>>>> DefaultSize to properly encode iret/fldenv/frstor/fsave/fstenv for
>>>>>>> .code16gcc, instead of checking i.tm.opcode_modifier.no_ssuf.
>>>>>>> Something like this.
>>>>>>
>>>>>> Plausible, but still afaict orthogonal to what I'm doing here, and
>>>>>> what you look to be unhappy about.
>>>>>
>>>>> DefaultSize only impacts .code16gcc.   Why adding DefaultSize to these
>>>>> instructions?
>>>>
>>>> There are two uses in process_suffix(), and only one of them is
>>>> .coge16gcc related afaict. The other also affects 64-bit mode,
>>>> or else I don't understand why various Cpu64 templates also have
>>>> the attribute.
>>>
>>> DefaultSize makes no difference on Cpu64 push/pop in AT&T syntax.
>>> It is only used by Intel syntax.  In AT&T syntax, only i.tm.opcode_modifier.w
>>> instructions need suffix.   I don't think we should add DefaultSize to more
>>> instructions.
>>
>> What I continue to miss is what you suggest as an alternative. Did
>> you mean to commit that other change, widening the attribute to 2
>> bits, and you'd then expect me to re-base over it? Aiui this would
>> improve the situation, but not necessarily avoid adding exceptions
>> (I'd have to check if it actually does). I'd also like to note that
>> your mention of only push/pop for 64-bit looks incomplete to me -
>> as said, call, ret, enter etc also have that attribute.
> 
> I don't think DefaultSize matters for them either in AT&T syntax.
> and I don't think we should add DefaultSize to more instructions.

Then _again_ - what is your alternative suggestion?

Jan