On Tue, May 12, 2009 at 11:30:21AM -0300, Marcelo Tosatti wrote:
> On Tue, May 12, 2009 at 10:13:36PM +0800, Yang, Sheng wrote:
> > > > + mutex_unlock(&kvm->lock);
> > >
> > > assigned_dev list is protected by kvm->lock. So you could have another
> > > ioctl adding to it at the same time you're searching.
> >
> > Oh, yes... My fault...
> >
> > > Could either have a separate kvm->assigned_devs_lock, to protect
> > > kvm->arch.assigned_dev_head (users are ioctls that manipulate it), or
> > > change the IRQ injection to use a separate spinlock, kill the workqueue
> > > and call kvm_set_irq from the assigned device interrupt handler.
> >
> > Peferred the latter, though needs more work. But the only reason for put a
> > workqueue here is because kvm->lock is a mutex? I can't believe... If so, I
> > think we had made a big mistake - we have to fix all kinds of racy problem
> > caused by this, but finally find it's unnecessary...
>
> One issue is that kvm_set_irq can take too long while interrupts are
> blocked, and you'd have to disable interrupts in other contexes that
> inject interrupts (say qemu->ioctl(SET_INTERRUPT)->...->), so all i can
> see is a tradeoff.
>
> <guess mode on>
>
> But the interrupt injection path seems to be pretty short and efficient
> to happen in host interrupt context.
>
> <guess mode off>
>
> Avi, Gleb?
>
> > Maybe another reason is kvm_kick_vcpu(), but have already fixed by you.
>
> Note you tested the spinlock_irq patch with GigE and there was no
> significant performance regression right?
>
> >
> > Continue to check the code...
OK, it might take some time for bigger changes to happen. I've changed
your patch to drop the lock only around cancel_work_sync. Can deadlock
if someone else tries to mess with the assigned device at the same time,
but the VM won't go away under it because of the vmfd reference.
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4d00942..ba067db 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -250,7 +250,15 @@ static void deassign_host_irq(struct kvm *kvm,
disable_irq_nosync(assigned_dev->
host_msix_entries[i].vector);
+ /*
+ * FIXME: kvm_assigned_dev_interrupt_work_handler can deadlock
+ * with cancel_work_sync, since it requires kvm->lock for irq
+ * injection. This is a hack, the irq code must use
+ * a separate lock.
+ */
+ mutex_unlock(&kvm->lock);
cancel_work_sync(&assigned_dev->interrupt_work);
+ mutex_lock(&kvm->lock);
for (i = 0; i < assigned_dev->entries_nr; i++)
free_irq(assigned_dev->host_msix_entries[i].vector,
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
