Date: Wed, 23 Nov 2005 22:13:53 +0100
From: Andi Kleen <ak@suse.de>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Linus Torvalds <torvalds@osdl.org>, "H. Peter Anvin" <hpa@zytor.com>,
       Andi Kleen <ak@suse.de>, Gerd Knorr <kraxel@suse.de>,
       Dave Jones <davej@redhat.com>, Zachary Amsden <zach@vmware.com>,
       Pavel Machek <pavel@ucw.cz>, Andrew Morton <akpm@osdl.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Zwane Mwaikambo <zwane@arm.linux.org.uk>,
       Pratap Subrahmanyam <pratap@vmware.com>,
       Christopher Li <chrisl@vmware.com>,
       "Eric W. Biederman" <ebiederm@xmission.com>,
       Ingo Molnar <mingo@elte.hu>
Subject: Re: [patch] SMP alternatives
Message-ID: <20051123211353.GR20775@brahms.suse.de>
References: <437B5A83.8090808@suse.de> <438359D7.7090308@suse.de> <p7364qjjhqx.fsf@verdi.suse.de> <1132764133.7268.51.camel@localhost.localdomain> <20051123163906.GF20775@brahms.suse.de> <1132766489.7268.71.camel@localhost.localdomain> <Pine.LNX.4.64.0511230858180.13959@g5.osdl.org> <4384AECC.1030403@zytor.com> <Pine.LNX.4.64.0511231031350.13959@g5.osdl.org> <1132782245.13095.4.camel@localhost.localdomain>
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On Wed, Nov 23, 2005 at 09:44:05PM +0000, Alan Cox wrote:
> On Mer, 2005-11-23 at 10:42 -0800, Linus Torvalds wrote:
> > Of course, if it's in one of the low 12 bits of %cr3, there would have to 
> > be a "enable this bit" in %cr4 or something. Historically, you could write 
> > any crap in the low bits, I think.
> 
> There is a much much better way to do it than just user space and
> without hitting cr3/cr4 - put "lock works" in the PAT and while we'll
> have to add PAT support which we need to do anyway we would get a world
> where on uniprocessor lock prefix only works on addresse targets we want
> it to - ie pci_alloc_consistent() pages.

The idea was to turn LOCK on only if the process has any
shared writable mapping and num_online_cpus() > 0.

Might be a bit costly to rewrite all the page tables for that case
just to change the PAT index.  A bit is nicer for that.

-Andi
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