From: Jim Westfall <jwestfall@surrealistic.net>
To: Jan Beulich <JBeulich@suse.com>
Cc: xen-devel@lists.xen.org
Subject: Re: fma4/avx under xen
Date: Fri, 4 May 2012 09:04:59 -0700 [thread overview]
Message-ID: <20120504160459.GX5985@surrealistic.net> (raw)
In-Reply-To: <4FA3BD2602000078000818F7@nat28.tlf.novell.com>
Jan Beulich <JBeulich@suse.com> wrote [05.04.12]:
> >>> On 02.05.12 at 20:54, Jim Westfall <jwestfall@surrealistic.net> wrote:
> > The summary is that when gcc compiles something with -mfma4 it also
> > enables the use of the avx instruction set. Since by default xen
> > disables avx it leads to invalid opcodes.
>
> Not that I know of, at least not anymore in current -unstable.
>
> > This ends up being kinda nasty with the multiarch glibc since its doing
> > stuff like
> >
> > (HAS_FMA4 ? run_fma4_func() : (HAS_AVX ? run_avx_func() : run_func()))
>
> The question is what (runtime) tests HAS_FMA includes. I know that
> some glibc versions had a broken AVX check (which only checked for
> the AVX feature flag, while the specification explicitly states that
> a prerequisite check of the OSXSAVE feature flag is also necessary).
Its just a basic cpuid() call and seeing if the FMA4 bit is set. I will
bug the glibc guys. Thanks for the feedback.
jim
prev parent reply other threads:[~2012-05-04 16:04 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-05-02 18:54 fma4/avx under xen Jim Westfall
2012-05-04 9:25 ` Ian Campbell
2012-05-04 9:27 ` Jan Beulich
2012-05-04 16:04 ` Jim Westfall [this message]
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