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xen-devel
[Xen-devel] Re: Question about x86/mm/gup.c's use of disabled interrupts
Avi Kivity wrote:
And the hypercall could result in no Xen-level IPIs at all, so it
could be very quick by comparison to an IPI-based Linux
implementation, in which case the flag polling would be particularly
harsh.
Maybe we could bring these optimizations into Linux as well. The only
thing Xen knows that Linux doesn't is if a vcpu is not scheduled; all
other information is shared.
I don't think there's a guarantee that just because a vcpu isn't running
now, it won't need a tlb flush. If a pcpu does runs vcpu 1 -> idle ->
vcpu 1, then there's no need for it to do a tlb flush, but the hypercall
can make force a flush when it reschedules vcpu 1 (if the tlb hasn't
already been flushed by some other means).
(I'm not sure to what extent Xen implements this now, but I wouldn't
want to over-constrain it.)
Also, the straightforward implementation of "poll until all target
cpu's flags are clear" may never make progress, so you'd have to
"scan flags, remove busy cpus from set, repeat until all cpus done".
All annoying because this race is pretty unlikely, and it seems a
shame to slow down all tlb flushes to deal with it. Some kind of
global "doing gup_fast" counter would get flush_tlb_others bypass the
check, at the cost of putting a couple of atomic ops around the
outside of gup_fast.
The nice thing about local_irq_disable() is that it scales so well.
Right. But it effectively puts the burden on the tlb-flusher to check
the state (implicitly, by trying to send an interrupt). Putting an
explicit poll in gets the same effect, but its pure overhead just to
deal with the gup race.
I'll put a patch together and see how it looks.
J
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