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Re: [Qemu-devel] [PATCH] softfloat: enforce softfloat if the host's FMA
From: |
Emilio G. Cota |
Subject: |
Re: [Qemu-devel] [PATCH] softfloat: enforce softfloat if the host's FMA is broken |
Date: |
Tue, 25 Dec 2018 01:55:39 -0500 |
User-agent: |
Mutt/1.9.4 (2018-02-28) |
On Mon, Dec 24, 2018 at 21:38:52 +0000, Peter Maydell wrote:
> On Mon, 24 Dec 2018 at 19:16, Emilio G. Cota <address@hidden> 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