Re: [PATCH v2 1/2] KVM: MMIO: Lock coalesced device when checking for available entry

Avi Kivity
2011-07-18T12:29:12Z

On 07/18/2011 03:03 PM, Sasha Levin wrote:
> On Mon, 2011-07-18 at 14:43 +0300, Avi Kivity wrote:
> > On 07/18/2011 01:15 PM, Sasha Levin wrote:
> > > On Mon, 2011-07-18 at 12:50 +0300, Avi Kivity wrote:
> > > > On 07/18/2011 12:29 PM, Sasha Levin wrote:
> > > > > > Hmm. This means we take the lock for every I/O, whether it hits
> > > > > > coalesced mmio or not.
> > > > > >
> > > > > > We need to do the range check before taking the lock and the space check
> > > > > > after taking the lock.
> > > > > >
> > > > >
> > > > > I'll fix that.
> > > > >
> > > > > Shouldn't the range check be also locked somehow? Currently it is
> > > > > possible that a coalesced region was removed while we are checking the
> > > > > ranges, and we won't issue a mmio exit as the host expects
> > > >
> > > > It's "locked" using rcu.
> > > >
> > >
> > > Where is that happening?
> > >
> > > All the coalesced zones are stored under the coalesced "device" in a
> > > simple array. When adding and removing zones, kvm->slots_lock is taken -
> > > I don't see anything which prevents a range check during zone removal
> > > unless slots_lock prevents IO.
> >
> > Range check during slot removal is legal. While you are removing a
> > slot, a concurrent write may hit or miss the slot; it doesn't matter.
> >
> > Userspace should flush the coalesced mmio buffer after removal to ensure
> > there are no pending writes.
> >
>
> But the write may hit a non-existent slot.
>
> Something like this:
>
> Thread 1 Thread 2
> ----------------------------------
> Check range |
> Found slot |
> | Remove slot
> | Flush buffer
> Get spinlock |
> Write to buffer |
>

Cannot happen, due to rcu. The "remove slot" step waits until all rcu
readers are gone.

In other words: it's magic.

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