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Re: [Xen-devel] [PATCH] xen/x86: Fix up rules when forcing -mno-sse

>>> On 12.08.15 at 18:12, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 12/08/15 17:03, Doug Goldstein wrote:
>> On Wed, Aug 12, 2015 at 10:38 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>>> On 12.08.15 at 17:23, <cardoe@xxxxxxxxxx> wrote:
>>>> From: Nathan Rossi <nathan.rossi@xxxxxxxxxx>
>>>> * When forcing -mno-sse specify -mfpmath=387 to avoid the fallback
>>>>   warning
>>> "the fallback warning" being what? I've never seen any, and I don't
>>> think I've ever heard anyone else say there was one.
>>> Apart from that I don't think -mfpmath=387 is what we want - we'd
>>> better tell the compiler to not use x87 registers at all.
> Agreed.
> However, a git grep shows:
> crypto/vmac.c:1098:    double cpb;
> crypto/vmac.c:1158:             
> ((double)CLOCKS_PER_SEC*speed_lengths[i]*speed_iters[i]));
> which I suspect might need fixing.

Not really - the grep didn't show you that this sits inside a main()
function inside #if VMAC_RUN_TESTS.

>> I guess I jumped the gun a bit. When targeting a corei7 based chip, Yocto 
>> uses
>> the following CFLAGS for the packages it builds:
>> -march=corei7 -mtune=corei7 -mfpmath=sse -msse4.2
>> The issue is the -mfpmath value.
> It seems wrong to set -mfpmath=387, as that implies that floating point
> maths is ok.
> I cant however find an obvious "no fpmath" option.
> Perhaps Xen should specifically filter options such as those out from
> the environmental CFLAGS

Maybe. But perhaps suppressing such being enforced by a build
system on a package like Xen (the hypervisor part of it, perhaps
for the tools this would be okay) or the kernel should rather be
done at the packaging level. Or we might consider ignoring CFLAGS
passed through the environment (and require overrides to be
done via make command line arguments instead).


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