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

On Thu, Aug 13, 2015 at 2:34 AM, Doug Goldstein <cardoe@xxxxxxxxxx> wrote:
> On Wed, Aug 12, 2015 at 11:12 AM, Andrew Cooper
> <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.

The fallback warning is just GCC getting conflicting -mfpmath options,
and falls back to 387 arithmetic (but the warning is treated as an
error). You may not see it with newer versions of GCC I have not
tested if the issue still persists. The specific message is:

"warning: SSE instruction set disabled, using 387 arithmetics"

Popped up as an issue with Yocto/OE + meta-virtualization layer. See

>>>> 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.

Using "-msoft-float -mno-fp-ret-in-387" might be useful, it will at
least generate linker errors (as the softfp library is not configured
to be linked) for all uses of float/double operations.

>> 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.
> Looks like that only comes in when VMAC_RUN_TESTS is defined. That code
> is inside a main().
> --
> Doug Goldstein

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