[Xen-devel] fma4/avx under xen

[Jim Westfall <jwestfall@xxxxxxxxxxxxxxxx>]

Hi

I wanted to bring this to your attention

https://bugs.launchpad.net/ubuntu/+source/eglibc/+bug/956051

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.

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()))

run_fma4_func() can end up bombing out from the invalid opcode when run 
under xen.

Its not clear to me if xen should be filtering fma4 as part of its avx 
filter or if gcc should not assume avx support when compiling with 
-mfma4.

thoughts?

thanks
jim

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