linux-kernel - Re: [RFC v2 8/8] x86, xsave: remove lazy allocation of xstate area

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Message-ID: <20110330131146.GO13@escobedo.osrc.amd.com>
Date:	Wed, 30 Mar 2011 15:11:46 +0200
From:	Hans Rosenfeld <hans.rosenfeld@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Brian Gerst <brgerst@...il.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...e.hu" <mingo@...e.hu>, "x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"suresh.b.siddha@...el.com" <suresh.b.siddha@...el.com>,
	"eranian@...gle.com" <eranian@...gle.com>,
	"Richter, Robert" <robert.richter@....com>,
	"Herrmann3, Andreas" <Andreas.Herrmann3@....com>
Subject: Re: [RFC v2 8/8] x86, xsave: remove lazy allocation of xstate area

On Tue, Mar 29, 2011 at 11:27:50AM -0400, H. Peter Anvin wrote:
> On 03/29/2011 07:17 AM, Hans Rosenfeld wrote:
> >>
> >> I'm not sure I like this.  I did a quick test on 64-bit, and found
> >> that while most if not all user processes allocated the fpu save area
> >> (probably because of glibc blindly initializing the fpu), kernel
> >> threads did not.  This patch would force kernel threads to allocate
> >> memory they would never use.
> > 
> > Yes, up to a few kilobytes would be wasted by kernel threads. The
> > related code gets much simpler. I think that is a good thing.
> > 
> 
> This is silly.  It shouldn't be very hard to allocate this for user
> threads while avoiding the allocation for kernel threads.  The only
> excuse for allocating it for user threads is if it becomes part of the
> kernel stack allocation.

The allocation itself is not what I'm concerned about. I'm more worried
about the code that always has to check whether a thread has a xstate
area allocated or not. But I will try and find out to get this done the
way you suggested.


Meanwhile, could you please review the other patches? They are much more
important to me.


Hans


-- 
%SYSTEM-F-ANARCHISM, The operating system has been overthrown

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