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From: Arnd Bergmann <arnd@arndb.de>
To: linuxppc-dev@ozlabs.org
Subject: Re: [RFC 2/3] PowerPC: lazy altivec enabling in kernel
Date: Tue, 24 Apr 2007 10:32:02 +0200
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Cc: Paul Mackerras <paulus@samba.org>
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On Tuesday 24 April 2007, Paul Mackerras wrote:
> It would be better to put a test and conditional branch in
> giveup_altivec to skip the mtmsrd if MSR_VEC is already set. =A0That
> would avoid adding the overhead of the trap in the case when MSR_VEC
> isn't already set, besides being much less code.

When I discussed this with Sebastian, my assumption was that even
the mfmsr is rather expensive by itself, but I may have interpreted
the profile data incorrectly.

Do you think it's safe to assume that by skipping mtmsr we can avoid
the bulk of the overhead on most CPUs?

	Arnd <><