Re: [patch] SMP alternatives

From: Andi Kleen
Date: Wed Nov 23 2005 - 16:13:51 EST

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.

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