Messages in this thread | | | Date | Wed, 18 Mar 2009 14:23:38 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: Question about x86/mm/gup.c's use of disabled interrupts |
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Avi Kivity wrote: > Jeremy Fitzhardinge wrote: >> Disabling the interrupt will prevent the tlb flush IPI from coming in >> and flushing this cpu's tlb, but I don't see how it will prevent some >> other cpu from actually updating the pte in the pagetable, which is >> what we're concerned about here. > > The thread that cleared the pte holds the pte lock and is now waiting > for the IPI. The thread that wants to update the pte will wait for > the pte lock, thus also waits on the IPI and gup_fast()'s > local_irq_enable(). I think.
But hasn't it already done the pte update at that point?
(I think this conversation really is moot because the kernel never does P->P pte updates any more; its always P->N->P.)
>> Is this the only reason to disable interrupts? > > Another comment says it also prevents pagetable teardown.
We could take a reference to the mm to get the same effect, no?
>> Also, assuming that disabling the interrupt is enough to get the >> guarantees we need here, there's a Xen problem because we don't use >> IPIs for cross-cpu tlb flushes (well, it happens within Xen). I'll >> have to think a bit about how to deal with that, but I'm thinking >> that we could add a per-cpu "tlb flushes blocked" flag, and maintain >> some kind of per-cpu deferred tlb flush count so we can get around to >> doing the flush eventually. > > I was thinking about adding a hypercall for cross-vcpu tlb flushes. > Guess I'll wait for you to clear up all the issues first.
Typical...
J
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