Re: [Qemu-devel] [RFC v2 7/8] virtio: event suppression for packed ring

[Jason Wang <jasowang@redhat.com>]

On 2018年06月06日 03:08, wexu@redhat.com wrote:
> From: Wei Xu <wexu@redhat.com>
>
> Signed-off-by: Wei Xu <wexu@redhat.com>
> Signed-off-by: Wei Xu <wexu@redhat.com>

Duplicated.

> ---
>   hw/virtio/virtio.c                             | 115 +++++++++++++++++++++++--
>   include/standard-headers/linux/virtio_config.h |  13 +++
>   2 files changed, 119 insertions(+), 9 deletions(-)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 6f2da83..4543974 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -226,6 +226,24 @@ static void vring_desc_read(VirtIODevice *vdev, VRingDesc *desc,
>       virtio_tswap16s(vdev, &desc->next);
>   }
>   
> +static void vring_packed_event_read(VirtIODevice *vdev,
> +                            MemoryRegionCache *cache, VRingPackedDescEvent *e)
> +{
> +    address_space_read_cached(cache, 0, e, sizeof(*e));
> +    virtio_tswap16s(vdev, &e->off_wrap);
> +    virtio_tswap16s(vdev, &e->flags);

You need to make sure flags is read before off_wrap.

> +}
> +
> +static void vring_packed_event_write(VirtIODevice *vdev,
> +                            MemoryRegionCache *cache, VRingPackedDescEvent *e)
> +{
> +    virtio_tswap16s(vdev, &e->off_wrap);
> +    virtio_tswap16s(vdev, &e->flags);
> +    address_space_write_cached(cache, 0, e, sizeof(*e));

You need make sure flags were wrote before off_wrap.

> +    address_space_cache_invalidate(cache, 0, sizeof(VRingUsedElem));
> +}
> +
> +
>   static VRingMemoryRegionCaches *vring_get_region_caches(struct VirtQueue *vq)
>   {
>       VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
> @@ -332,14 +350,8 @@ static inline void vring_set_avail_event(VirtQueue *vq, uint16_t val)
>       address_space_cache_invalidate(&caches->used, pa, sizeof(val));
>   }
>   
> -void virtio_queue_set_notification(VirtQueue *vq, int enable)
> +static void virtio_queue_set_notification_split(VirtQueue *vq, int enable)
>   {
> -    vq->notification = enable;
> -
> -    if (!vq->vring.desc) {
> -        return;
> -    }
> -
>       rcu_read_lock();
>       if (virtio_vdev_has_feature(vq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
>           vring_set_avail_event(vq, vring_avail_idx(vq));
> @@ -355,6 +367,38 @@ void virtio_queue_set_notification(VirtQueue *vq, int enable)
>       rcu_read_unlock();
>   }
>   
> +static void virtio_queue_set_notification_packed(VirtQueue *vq, int enable)
> +{
> +    VRingPackedDescEvent e;
> +    VRingMemoryRegionCaches *caches;
> +
> +    rcu_read_lock();
> +    caches  = vring_get_region_caches(vq);
> +    vring_packed_event_read(vq->vdev, &caches->device, &e);

Why need read?

> +    if (enable) {
> +        e.flags = RING_EVENT_FLAGS_ENABLE;
> +    } else {
> +        e.flags = RING_EVENT_FLAGS_DISABLE;
> +    }
> +    vring_packed_event_write(vq->vdev, &caches->device, &e);
> +    rcu_read_unlock();
> +}
> +
> +void virtio_queue_set_notification(VirtQueue *vq, int enable)
> +{
> +    vq->notification = enable;
> +
> +    if (!vq->vring.desc) {
> +        return;
> +    }
> +
> +    if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
> +        virtio_queue_set_notification_packed(vq, enable);
> +    } else {
> +        virtio_queue_set_notification_split(vq, enable);
> +    }
> +}
> +
>   int virtio_queue_ready(VirtQueue *vq)
>   {
>       return vq->vring.avail != 0;
> @@ -2059,8 +2103,7 @@ static void virtio_set_isr(VirtIODevice *vdev, int value)
>       }
>   }
>   
> -/* Called within rcu_read_lock().  */
> -static bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
> +static bool virtio_split_should_notify(VirtIODevice *vdev, VirtQueue *vq)
>   {
>       uint16_t old, new;
>       bool v;
> @@ -2083,6 +2126,60 @@ static bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
>       return !v || vring_need_event(vring_get_used_event(vq), new, old);
>   }
>   
> +static bool vring_packed_need_event(VirtQueue *vq, uint16_t off_wrap,
> +                                    uint16_t new, uint16_t old)
> +{
> +    bool wrap = vq->used_wrap_counter;
> +    int off = off_wrap & ~(1 << 15);
> +
> +    if (new < old) {
> +        new += vq->vring.num;
> +        wrap ^= 1;
> +    }
> +
> +    if (wrap != off_wrap >> 15) {
> +        off += vq->vring.num;
> +    }
> +
> +    return vring_need_event(off, new, old);
> +}
> +
> +static bool virtio_packed_should_notify(VirtIODevice *vdev, VirtQueue *vq)
> +{
> +    VRingPackedDescEvent e;
> +    uint16_t old, new;
> +    bool v;
> +    VRingMemoryRegionCaches *caches;
> +
> +    caches  = vring_get_region_caches(vq);
> +    vring_packed_event_read(vdev, &caches->driver, &e);
> +
> +    /* Make sure we see the updated flags */
> +    smp_mb();

Like I mention several times, why need a memory barrier here?

> +    if (e.flags == RING_EVENT_FLAGS_DISABLE) {
> +        return false;
> +    } else if (e.flags == RING_EVENT_FLAGS_ENABLE) {
> +        return true;
> +    }
> +
> +    v = vq->signalled_used_valid;
> +    vq->signalled_used_valid = true;
> +    old = vq->signalled_used;
> +    new = vq->signalled_used = vq->used_idx;
> +
> +    return !v || vring_packed_need_event(vq, e.off_wrap, new, old);
> +}
> +
> +/* Called within rcu_read_lock().  */
> +static bool virtio_should_notify(VirtIODevice *vdev, VirtQueue *vq)
> +{
> +    if (virtio_vdev_has_feature(vdev, VIRTIO_F_RING_PACKED)) {
> +        return virtio_packed_should_notify(vdev, vq);
> +    } else {
> +        return virtio_split_should_notify(vdev, vq);
> +    }
> +}
> +
>   void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
>   {
>       bool should_notify;
> diff --git a/include/standard-headers/linux/virtio_config.h b/include/standard-headers/linux/virtio_config.h
> index 6ee5529..53e5c83 100644
> --- a/include/standard-headers/linux/virtio_config.h
> +++ b/include/standard-headers/linux/virtio_config.h
> @@ -73,4 +73,17 @@
>   #define VIRTIO_F_IOMMU_PLATFORM		33
>   
>   #define VIRTIO_F_RING_PACKED		34
> +
> +/* Enable events */
> +#define RING_EVENT_FLAGS_ENABLE 0x0
> +/* Disable events */
> +#define RING_EVENT_FLAGS_DISABLE 0x1
> +/*
> + *  * Enable events for a specific descriptor
> + *   * (as specified by Descriptor Ring Change Event Offset/Wrap Counter).
> + *    * Only valid if VIRTIO_F_RING_EVENT_IDX has been negotiated.
> + *     */
> +#define RING_EVENT_FLAGS_DESC 0x2
> +/* The value 0x3 is reserved */
> +
>   #endif /* _LINUX_VIRTIO_CONFIG_H */

This could be done in a patch of header sync.

Thanks