Re: [RFC PATCH 06/27] virtio: Add virtio_queue_get_used_notify_split

[Stefan Hajnoczi]

On Fri, Nov 20, 2020 at 07:50:44PM +0100, Eugenio Pérez wrote:
> This function is just used for a few commits, so SW LM is developed
> incrementally, and it is deleted after it is useful.
> 
> For a few commits, only the events (irqfd, eventfd) are forwarded.

s/eventfd/ioeventfd/ (irqfd is also an eventfd)

> +bool virtio_queue_get_used_notify_split(VirtQueue *vq)
> +{
> +    VRingMemoryRegionCaches *caches;
> +    hwaddr pa = offsetof(VRingUsed, flags);
> +    uint16_t flags;
> +
> +    RCU_READ_LOCK_GUARD();
> +
> +    caches = vring_get_region_caches(vq);
> +    assert(caches);
> +    flags = virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);
> +    return !(VRING_USED_F_NO_NOTIFY & flags);
> +}

QEMU stores the notification status:

void virtio_queue_set_notification(VirtQueue *vq, int enable)
{
    vq->notification = enable; <---- here

    if (!vq->vring.desc) {
        return;
    }

    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
        virtio_queue_packed_set_notification(vq, enable);
    } else {
        virtio_queue_split_set_notification(vq, enable);

I'm wondering why it's necessary to fetch from guest RAM instead of
using vq->notification? It also works for both split and packed
queues so the code would be simpler.

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