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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.10.17 at 17:05, <george.dunlap@xxxxxxxxxx> wrote:
> On 10/16/2017 01:32 PM, 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). 
> 
> Why doesn't this affect the rest of the hypervisor too, since we don't
> save and restore the *mm registers?

Because we build the hypervisor with -mno-sse.

>> 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.
> 
> Sorry, just to clarify: You mean that there are standard headers which
> contain prototypes for functions which return floating point values; we
> include those headers but do not call the functions; and adding the
> #pragma to the command-line would cause the compiler to choke on the
> prototypes (even though the functions are never actually called)?

Yes (adding the command line option equivalent of the pragma,
that is).

Jan


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