Re: [PATCH v2 1/2] KVM: MMIO: Lock coalesced device when checking for available entry

Avi Kivity
2011-07-18T08:12:13Z

On 07/15/2011 02:37 PM, Sasha Levin wrote:
> Move the check whether there are available entries to within the spinlock.
> This allows working with larger amount of VCPUs and reduces premature
> exits when using a large number of VCPUs.
>
> Cc: Avi Kivity<avi@redhat.com>
> Cc: Ingo Molnar<mingo@elte.hu>
> Cc: Marcelo Tosatti<mtosatti@redhat.com>
> Cc: Pekka Enberg<penberg@kernel.org>
> Signed-off-by: Sasha Levin<levinsasha928@gmail.com>
> ---
> virt/kvm/coalesced_mmio.c | 9 ++++++---
> 1 files changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
> index fc84875..34188db 100644
> --- a/virt/kvm/coalesced_mmio.c
> +++ b/virt/kvm/coalesced_mmio.c
> @@ -37,7 +37,7 @@ static int coalesced_mmio_in_range(struct kvm_coalesced_mmio_dev *dev,
> */
> ring = dev->kvm->coalesced_mmio_ring;
> avail = (ring->first - ring->last - 1) % KVM_COALESCED_MMIO_MAX;
> - if (avail< KVM_MAX_VCPUS) {
> + if (avail == 0) {
> /* full */
> return 0;
> }
> @@ -63,11 +63,14 @@ static int coalesced_mmio_write(struct kvm_io_device *this,
> {
> struct kvm_coalesced_mmio_dev *dev = to_mmio(this);
> struct kvm_coalesced_mmio_ring *ring = dev->kvm->coalesced_mmio_ring;
> - if (!coalesced_mmio_in_range(dev, addr, len))
> - return -EOPNOTSUPP;
>
> spin_lock(&dev->lock);
>
> + if (!coalesced_mmio_in_range(dev, addr, len)) {
> + spin_unlock(&dev->lock);
> + return -EOPNOTSUPP;
> + }
> +
> /* copy data in first free entry of the ring */

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.

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