As of commit 38738f7dbbda90fbc161757b7f4be35b52205552 ("virtio-scsi:
don't waste CPU polling the event virtqueue"), we only attach an io_read
notifier for the virtio-scsi event virtqueue instead, and no polling
notifiers.  During operation, the event virtqueue is typically
non-empty, but none of the buffers are intended to be used immediately.
Instead, they only get used when certain events occur.  Therefore, it
makes no sense to continuously poll it when non-empty, because it is
supposed to be and stay non-empty.

We do this by using virtio_queue_aio_attach_host_notifier_no_poll()
instead of virtio_queue_aio_attach_host_notifier() for the event
virtqueue.

Commit 766aa2de0f29b657148e04599320d771c36fd126 ("virtio-scsi: implement
BlockDevOps->drained_begin()") however has virtio_scsi_drained_end() use
virtio_queue_aio_attach_host_notifier() for all virtqueues, including
the event virtqueue.  This can lead to it being polled again, undoing
the benefit of commit 38738f7dbbda90fbc161757b7f4be35b52205552.

Fix it by using virtio_queue_aio_attach_host_notifier_no_poll() for the
event virtqueue.

Reported-by: Fiona Ebner <f.ebner@proxmox.com>
Fixes: 766aa2de0f29b657148e04599320d771c36fd126
       ("virtio-scsi: implement BlockDevOps->drained_begin()")
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 hw/scsi/virtio-scsi.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 690aceec45..9f02ceea09 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -1149,6 +1149,7 @@ static void virtio_scsi_drained_begin(SCSIBus *bus)
 static void virtio_scsi_drained_end(SCSIBus *bus)
 {
     VirtIOSCSI *s = container_of(bus, VirtIOSCSI, bus);
+    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(s);
     VirtIODevice *vdev = VIRTIO_DEVICE(s);
     uint32_t total_queues = VIRTIO_SCSI_VQ_NUM_FIXED +
                             s->parent_obj.conf.num_queues;
@@ -1166,7 +1167,11 @@ static void virtio_scsi_drained_end(SCSIBus *bus)
 
     for (uint32_t i = 0; i < total_queues; i++) {
         VirtQueue *vq = virtio_get_queue(vdev, i);
-        virtio_queue_aio_attach_host_notifier(vq, s->ctx);
+        if (vq == vs->event_vq) {
+            virtio_queue_aio_attach_host_notifier_no_poll(vq, s->ctx);
+        } else {
+            virtio_queue_aio_attach_host_notifier(vq, s->ctx);
+        }
     }
 }
 
-- 
2.43.0

Am 24.01.24 um 18:38 schrieb Hanna Czenczek:
> As of commit 38738f7dbbda90fbc161757b7f4be35b52205552 ("virtio-scsi:
> don't waste CPU polling the event virtqueue"), we only attach an io_read
> notifier for the virtio-scsi event virtqueue instead, and no polling
> notifiers.  During operation, the event virtqueue is typically
> non-empty, but none of the buffers are intended to be used immediately.
> Instead, they only get used when certain events occur.  Therefore, it
> makes no sense to continuously poll it when non-empty, because it is
> supposed to be and stay non-empty.
> 
> We do this by using virtio_queue_aio_attach_host_notifier_no_poll()
> instead of virtio_queue_aio_attach_host_notifier() for the event
> virtqueue.
> 
> Commit 766aa2de0f29b657148e04599320d771c36fd126 ("virtio-scsi: implement
> BlockDevOps->drained_begin()") however has virtio_scsi_drained_end() use
> virtio_queue_aio_attach_host_notifier() for all virtqueues, including
> the event virtqueue.  This can lead to it being polled again, undoing
> the benefit of commit 38738f7dbbda90fbc161757b7f4be35b52205552.
> 
> Fix it by using virtio_queue_aio_attach_host_notifier_no_poll() for the
> event virtqueue.
> 
> Reported-by: Fiona Ebner <f.ebner@proxmox.com>
> Fixes: 766aa2de0f29b657148e04599320d771c36fd126
>        ("virtio-scsi: implement BlockDevOps->drained_begin()")
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>

Tested-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>

On Wed, Jan 24, 2024 at 06:38:29PM +0100, Hanna Czenczek wrote:
> As of commit 38738f7dbbda90fbc161757b7f4be35b52205552 ("virtio-scsi:
> don't waste CPU polling the event virtqueue"), we only attach an io_read
> notifier for the virtio-scsi event virtqueue instead, and no polling
> notifiers.  During operation, the event virtqueue is typically
> non-empty, but none of the buffers are intended to be used immediately.
> Instead, they only get used when certain events occur.  Therefore, it
> makes no sense to continuously poll it when non-empty, because it is
> supposed to be and stay non-empty.
> 
> We do this by using virtio_queue_aio_attach_host_notifier_no_poll()
> instead of virtio_queue_aio_attach_host_notifier() for the event
> virtqueue.
> 
> Commit 766aa2de0f29b657148e04599320d771c36fd126 ("virtio-scsi: implement
> BlockDevOps->drained_begin()") however has virtio_scsi_drained_end() use
> virtio_queue_aio_attach_host_notifier() for all virtqueues, including
> the event virtqueue.  This can lead to it being polled again, undoing
> the benefit of commit 38738f7dbbda90fbc161757b7f4be35b52205552.
> 
> Fix it by using virtio_queue_aio_attach_host_notifier_no_poll() for the
> event virtqueue.
> 
> Reported-by: Fiona Ebner <f.ebner@proxmox.com>
> Fixes: 766aa2de0f29b657148e04599320d771c36fd126
>        ("virtio-scsi: implement BlockDevOps->drained_begin()")
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> ---
>  hw/scsi/virtio-scsi.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

Thank you!

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>