On Sun, May 24, 2009 at 05:04:52PM +0300, Avi Kivity wrote: > Marcelo Tosatti wrote: >> Get rid of kvm->lock dependency on coalesced_mmio methods. Use an >> atomic variable instead to guarantee only one vcpu is batching >> data into the ring at a given time. >> >> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> >> >> Index: kvm-irqlock/virt/kvm/coalesced_mmio.c >> =================================================================== >> --- 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? 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? Agree with the suggestion on patch 3, will fix the commentary on kvm_host.h. -- 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
