On Wed, May 20, 2009 at 12:13:03PM -0300, Marcelo Tosatti wrote: > On Wed, May 20, 2009 at 05:29:23PM +0300, Avi Kivity wrote: > > Marcelo Tosatti wrote: > > > > > > > >>> So we have a function that takes a lock and conditionally releases it? > >>> > >> > >> Yes, but it is correct: it will only return with the lock held in case > >> it returns 1, in which case its guaranteed ->write will be called (which > >> will unlock it). > >> > >> It should check the range first and/or use some smarter synchronization, > >> but one thing at a time. > >> > > > > Yes it's correct but we'll get an endless stream of patches to 'fix' it > > because it is so unorthodox. > > Does it have to guarantee any kind of ordering in case of parallel > writes by distincting vcpus? This is what it does now (so if a vcpu > arrives first, the second will wait until the first is finished > processing). > > I suppose that is the responsability of the guest (if it does MMIO > writes to a device in parallel it'll screwup in real HW too). > > Because in such case, you can drop the mutex and protect only the kvm > data. If you want it to provide ordering (as in process the MMIO writes, by multiple vcpus, in the order they happen), you need a mutex or spinlock. And in that case, I don't see any other way around this given the way ->in_range / ->read/->write work. Even if you change the order in which the full mmio buffer check and coalesced-allowed-in-this-range happen you still need a spinlock/mutex in this unorthodox way. -- 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
