Re: kern/50332: AVX instructions don't work but OSXSAVE flag is set

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

The following reply was made to PR kern/50332; it has been noted by GNATS.

From: Manuel Bouyer <bouyer%antioche.eu.org@localhost>
To: =?iso-8859-1?Q?Torbj=F6rn?= Granlund <tg%gmplib.org@localhost>
Cc: gnats-bugs%netbsd.org@localhost, kern-bug-people%netbsd.org@localhost, gnats-admin%netbsd.org@localhost,
        netbsd-bugs%netbsd.org@localhost
Subject: Re: kern/50332: AVX instructions don't work but OSXSAVE flag is set
Date: Tue, 20 Oct 2015 21:23:48 +0200

 On Tue, Oct 20, 2015 at 06:41:54PM +0200, Torbjörn Granlund wrote:
 > David Laight <david%l8s.co.uk@localhost> writes:
 > 
 >    No, there is no reason why you can't have a cpu (and os) that supports xsave
 >    (etc) but doesn't support avx.
 >    You have to look at the bitmap of supported extensions.
 >    IIRC Only the x87 fpu registers are mandatory.
 >    
 > Note that my updated test case tests both flags to make this a moot
 > point.
 > 
 >    However that probably isn't the issue here.
 >    More likely is that something in xen should stop avx being used,
 >    or xen needs to do something different from bare-metal to support avx.
 >    
 > Please note that non-NetBSD guest systems have no problem.  Also, NetBSD
 > with no Xen around works fine.
 > 
 > That makes me suspect Xen is innocent and that NetBSD gets this wrong
 > when run with Xen.
 > 
 > My complaint is not that AVX does not work under Xen+NetBSD, but that it
 > doesn't work in spite of that NetBSD sets OSXSAVE.
 
 AFAIK NetBSD is not setting OSXSAVE. This comes from the cpuid instruction,
 and I don't think the OS can change what cpuid repports.
 
 -- 
 Manuel Bouyer <bouyer%antioche.eu.org@localhost>
      NetBSD: 26 ans d'experience feront toujours la difference
 --