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Re: [Qemu-devel] [PATCH V3 09/14] virtio: introduce vector to virtqueues

From: Jason Wang
Subject: Re: [Qemu-devel] [PATCH V3 09/14] virtio: introduce vector to virtqueues mapping
Date: Mon, 09 Mar 2015 15:41:44 +0800

On Fri, Mar 6, 2015 at 8:55 PM, Cornelia Huck <address@hidden> wrote:
On Thu,  5 Mar 2015 13:48:46 +0800
Jason Wang <address@hidden> wrote:

Currently we will try to traverse all virtqueues to find a subset that
 using a specific vector. This is sub optimal when we will support
 hundreds or even thousands of virtqueues. So this patch introduces a
 method which could be used by transport to get all virtqueues that
 using a same vector. This is done through QLISTs and the number of
 QLISTs was queried through a transport specific method. When guest
setting vectors, the virtqueue will be linked and helpers for traverse
 the list was also introduced.
The first user will be virtio pci which will use this to speed up
 MSI-X masking and unmasking handling.

Will there be any users beyond virtio-pci, though?

Currently not. But if the vector could be shared in another transport, this is still useful.
 For virtio-ccw, at
least, "vectors" are an identity mapping of the queue index. I'm not
sure if introducing (memory) overhead for everyone is worth it.

I see. Another reason I make it generic is because I believe we try hard to not expose the internals of VirtQueue to transport specific code.

Cc: Anthony Liguori <address@hidden>
 Cc: Michael S. Tsirkin <address@hidden>
 Signed-off-by: Jason Wang <address@hidden>
  hw/virtio/virtio-pci.c         |  8 ++++++++
hw/virtio/virtio.c | 32 ++++++++++++++++++++++++++++++--
  include/hw/virtio/virtio-bus.h |  1 +
  include/hw/virtio/virtio.h     |  3 +++
  4 files changed, 42 insertions(+), 2 deletions(-)

void virtio_queue_set_vector(VirtIODevice *vdev, int n, uint16_t vector)
 -    if (n < virtio_get_queue_max(vdev))
 +    VirtQueue *vq = &vdev->vq[n];
 +    if (n < virtio_get_queue_max(vdev)) {
 +        if (vdev->vq[n].vector != VIRTIO_NO_VECTOR) {
 +            QLIST_REMOVE(vq, node);
 +        }
          vdev->vq[n].vector = vector;
 +        if (vector != VIRTIO_NO_VECTOR) {
+ QLIST_INSERT_HEAD(&vdev->vector_queues[vector], vq, node);
 +        }
 +    }

Is there any way to remove an entry?

Yes, e.g setting the vector to VIRTIO_NO_VECTOR (e.g during reset or driver exit).
 E.g., if the guest unassociates
virtqueues. (I just noticed I probably need to use VIRTIO_NO_VECTOR
instead of 0 for that case in ccw.)

Yes, I see 0 is used in virtio_ccw_set_vqs() when addr is zero. If there's no special consideration here, need to use VIRITO_NO_VECTOR.

Or maybe I'm completely misunderstanding what vectors are doing on
pci :)

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