Re: [patch 2/4] KVM: move coalesced_mmio locking to its own device

Avi Kivity
2009-05-24T14:04:54Z

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@redhat.com>
>
> 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!)

Suggest our own lock here. in_use is basically a homemade lock, better
to use the factory made ones which come with a warranty.

--
error compiling committee.c: too many arguments to function