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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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).

Jan


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