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. -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
