On 11 February 2011 12:24, Russell King - ARM Linux
[off-list ref] wrote:

On Fri, Feb 11, 2011 at 12:12:25PM +0000, Catalin Marinas wrote:

quoted

On SMP systems, we save the VFP at every context switch to deal with the
thread migration (though I have a plan to make this lazily on SMP as
well).

I'm not sure it's worth the complexity. ?You'd have to do an IPI to the
old CPU to provoke it to save the context from its VFP unit. ?You'd have
to do that in some kind of atomic way as the old CPU may be in the middle
of already saving it. ?You're also going to have to add locking to the
last_VFP_context[] array as other CPUs will be accessing non-local
entries, and that means doing locking in assembly. ?Yuck.

I wasn't thinking about that, too complex indeed. But it may be easier
to detect thread migration, possibly with some hooks into generic
scheduler and only save the VFP state at that point. I haven't looked
in detail but I heard the x86 people have patches for something
similar.

-- 
Catalin