[Xen-devel] Ping#2: [PATCH] x86emul: keep compiler from using {x, y, z}mm registers itself

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.11.17 at 16:04, <george.dunlap@xxxxxxxxxx> wrote:
> On 11/06/2017 11:59 AM, Jan Beulich wrote:
>>>>> On 16.10.17 at 14:42,  wrote:
>>>>>> On 16.10.17 at 14:37, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> On 16/10/17 13:32, Jan Beulich wrote:
>>>>> Since the emulator acts on the live hardware registers, we need to
>>>>> prevent the compiler from using them e.g. for inlined memcpy() /
>>>>> memset() (as gcc7 does). We can't, however, set this from the command
>>>>> line, as otherwise the 64-bit build would face issues with functions
>>>>> returning floating point values and being declared in standard headers.
>>>>>
>>>>> As the pragma isn't available prior to gcc6, we need to invoke it
>>>>> conditionally. Luckily up to gcc6 we haven't seen generated code access
>>>>> SIMD registers beyond what our asm()s do.
>>>>>
>>>>> Reported-by: George Dunlap <george.dunlap@xxxxxxxxxx>
>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>> ---
>>>>> While this doesn't affect core functionality, I think it would still be
>>>>> nice for it to be allowed in for 4.10.
>>>>
>>>> Agreed.
>>>>
>>>> Has this been tested with Clang?
>>>
>>> Sorry, no - still haven't got around to set up a suitable Clang
>>> locally.
>>>
>>>>  It stands a good chance of being
>>>> compatible, but we may need an && !defined(__clang__) included.
>>>
>>> Should non-gcc silently ignore "#pragma GCC ..." it doesn't
>>> recognize, or not define __GNUC__ in the first place if it isn't
>>> sufficiently compatible? I.e. if anything I'd expect we need
>>> "#elif defined(__clang__)" to achieve the same for Clang by
>>> some different pragma (if such exists).
>> 
>> Not having received any reply so far, I'm wondering whether
>> being able to build the test harness with clang is more
>> important than for it to work correctly when built with gcc. I
>> can't predict when I would get around to set up a suitable
>> clang on my dev systems.
> 
> I agree with the argument you make above.  On the unlikely chance
> there's a problem Travis should catch it, and someone who actually has a
> clang setup can help sort it out.

I'm still lacking an ack, before it being sensible to check with Julien
whether this is still fine to go in at this late stage.

Jan


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