On Tue, May 26, 2009 at 02:24:33PM +0300, Avi Kivity wrote: > Marcelo Tosatti wrote: >>>> =================================================================== >>>> --- kvm-irqlock.orig/virt/kvm/coalesced_mmio.c >>>> +++ kvm-irqlock/virt/kvm/coalesced_mmio.c >>>> @@ -26,9 +26,12 @@ static int coalesced_mmio_in_range(struc >>>> if (!is_write) >>>> return 0; >>>> - /* kvm->lock is taken by the caller and must be not released before >>>> - * dev.read/write >>>> - */ >>>> + /* >>>> + * Some other vcpu might be batching data into the ring, >>>> + * fallback to userspace. Ordering not our problem. >>>> + */ >>>> + if (!atomic_add_unless(&dev->in_use, 1, 1)) >>>> + return 0; >>>> >>> Ordering with simultaneous writes is indeed not our problem, but the >>> ring may contain ordered writes (even by the same vcpu!) >>> >> >> You mean writes by a vcpu should maintain ordering even when that vcpu >> migrates to a different pcpu? > > No. Writes by a single vcpu need to preserve their ordering relative to > each other. If a write by vcpu A completes before a write by vcpu B > starts, then they need to be ordered, since the vcpus may have > synchronized in between. > > Hm, I don't thimk you broke these rules. > >> The smp_wmb in coalesced_write guarantees that. >> >> >>> Suggest our own lock here. in_use is basically a homemade lock, >>> better to use the factory made ones which come with a warranty. >>> >> >> The first submission had a mutex but was considered unorthodox. Although >> i agree with your argument made then, i don't see a way around that. >> >> Some pseudocode please? > > Why not use slots_lock to protect the entire iodevice list (rcu one > day), and an internal spinlock for coalesced mmio? Don't like using slots_lock to protect the entire iodevice list, its reverse progress in my opinion. The PIO/MMIO device lists are data structures used in hot path, they are not directly related to the memslots, and therefore deserve a separate lock. Whenever slots_lock becomes an issue, you'll have to entangle the mess, so better start doing it now. Sure can switch to a internal spinlock for coalesced_mmio, but breaking out if spin_trylock fails. You're OK with that? -- 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
