virtio: read avail_idx from VQ only when necessary
The virtqueue_pop() implementation needs to check if the avail ring contains some pending buffers. To perform this check, it is not always necessary to fetch the avail_idx in the VQ memory, which is expensive. This patch introduces a shadow variable tracking avail_idx and modifies virtio_queue_empty() to access avail_idx in physical memory only when necessary. Signed-off-by: NVincenzo Maffione <v.maffione@gmail.com> Message-Id: <b617d6459902773d9f4ab843bfaca764f5af8eda.1450218353.git.v.maffione@gmail.com> Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com> Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com> Reviewed-by: NMichael S. Tsirkin <mst@redhat.com> Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
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