Re: [Qemu-devel] [PATCH] softfloat: enforce softfloat if the host's FMA is broken

["Emilio G. Cota" <cota@braap.org>]

On Mon, Dec 24, 2018 at 21:38:52 +0000, Peter Maydell wrote:
> On Mon, 24 Dec 2018 at 19:16, Emilio G. Cota <cota@braap.org> wrote:
> > The added branch to the FMA ops is marked as unlikely and therefore
> > its impact on performance (measured with fp-bench) is within noise range
> > when measured on an Intel(R) Xeon(R) Gold 6142 CPU @ 2.60GHz.
> 
> > +        /* check whether avx is usable */
> > +        if (c & bit_OSXSAVE) {
> > +            int bv;
> > +
> > +            __asm("xgetbv" : "=a"(bv), "=d"(d) : "c"(0));
> > +            if ((bv & 6) == 6) {
> > +                has_avx = c & bit_AVX;
> > +            }
> > +        }
> 
> I note that the code in tcg/i386/tcg-target.inc.c
> that does something similar has the note:
>             /* The xgetbv instruction is not available to older versions of
>              * the assembler, so we encode the instruction manually.
>              */
>  -- does that concern apply here?

Yes.

> This will also be the fourth piece of code that has an
> inline xgetbv asm insn to try to query a property of the
> CPU (the other 3 are target/i386/hvf/x86_cpuid.c, util/bufferiszero.c
> and tcg/i386/tcg-target.inc.c), which strongly suggests that we
> should abstract this out a bit better. That also might help
> avoid bugs which exist in one version but not the other:
> see for instance https://bugs.launchpad.net/bugs/1758819

Agreed.

I'll be away until mid-January starting tomorrow, so I'll
send a v2 with the cpuid bits removed, which aren't needed
to fix the bug that Laurent hit.

We can later on address cpuid consolidation.

Thanks,

		Emilio