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

[Avi Kivity <avi@redhat.com>]

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