linux-kernel - Re: [PATCH 1/2] x86: eliminate TS

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Date:	Sun, 2 May 2010 13:38:41 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Dexuan Cui <dexuan.cui@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 1/2] x86: eliminate TS_XSAVE

On Sun, May 2, 2010 at 10:53 AM, Avi Kivity <avi@...hat.com> wrote:
> The fpu code currently uses current->thread_info->status & TS_XSAVE as
> a way to distinguish between XSAVE capable processors and older processors.
> The decision is not really task specific; instead we use the task status to
> avoid a global memory reference - the value should be the same across all
> threads.
>
> Eliminate this tie-in into the task structure by using an alternative
> instruction keyed off the XSAVE cpu feature; this results in shorter and
> faster code, without introducing a global memory reference.

I think you should either just use cpu_has_xsave, or extend this use
of alternatives to all cpu features.  It doesn't make sense to only do
it for xsave.

--
Brian Gerst
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