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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/11/17 13:26, Jan Beulich wrote:
>>>> 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.

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

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