lkml.org 
[lkml]   [2009]   [Mar]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Question about x86/mm/gup.c's use of disabled interrupts
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



\
 
 \ /
  Last update: 2009-03-18 22:27    [W:0.097 / U:1.272 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site