Thu, Jun 20, 2024 at 12:31:34PM CEST, [email protected] wrote:
>On Thu, 20 Jun 2024 06:11:40 -0400, "Michael S. Tsirkin" <[email protected]>
>wrote:
>> On Thu, Jun 20, 2024 at 06:10:51AM -0400, Michael S. Tsirkin wrote:
>> > On Thu, Jun 20, 2024 at 05:53:15PM +0800, Heng Qi wrote:
>> > > On Thu, 20 Jun 2024 16:26:05 +0800, Jason Wang <[email protected]>
>> > > wrote:
>> > > > On Thu, Jun 20, 2024 at 4:21 PM Jason Wang <[email protected]> wrote:
>> > > > >
>> > > > > On Thu, Jun 20, 2024 at 3:35 PM Heng Qi <[email protected]>
>> > > > > wrote:
>> > > > > >
>> > > > > > On Wed, 19 Jun 2024 17:19:12 -0400, "Michael S. Tsirkin"
>> > > > > > <[email protected]> wrote:
>> > > > > > > On Thu, Jun 20, 2024 at 12:19:05AM +0800, Heng Qi wrote:
>> > > > > > > > @@ -5312,7 +5315,7 @@ static int virtnet_find_vqs(struct
>> > > > > > > > virtnet_info *vi)
>> > > > > > > >
>> > > > > > > > /* Parameters for control virtqueue, if any */
>> > > > > > > > if (vi->has_cvq) {
>> > > > > > > > - callbacks[total_vqs - 1] = NULL;
>> > > > > > > > + callbacks[total_vqs - 1] = virtnet_cvq_done;
>> > > > > > > > names[total_vqs - 1] = "control";
>> > > > > > > > }
>> > > > > > > >
>> > > > > > >
>> > > > > > > If the # of MSIX vectors is exactly for data path VQs,
>> > > > > > > this will cause irq sharing between VQs which will degrade
>> > > > > > > performance significantly.
>> > > > > > >
>> > > > >
>> > > > > Why do we need to care about buggy management? I think libvirt has
>> > > > > been teached to use 2N+2 since the introduction of the multiqueue[1].
>> > > >
>> > > > And Qemu can calculate it correctly automatically since:
>> > > >
>> > > > commit 51a81a2118df0c70988f00d61647da9e298483a4
>> > > > Author: Jason Wang <[email protected]>
>> > > > Date: Mon Mar 8 12:49:19 2021 +0800
>> > > >
>> > > > virtio-net: calculating proper msix vectors on init
>> > > >
>> > > > Currently, the default msix vectors for virtio-net-pci is 3 which
>> > > > is
>> > > > obvious not suitable for multiqueue guest, so we depends on the
>> > > > user
>> > > > or management tools to pass a correct vectors parameter. In fact,
>> > > > we
>> > > > can simplifying this by calculating the number of vectors on
>> > > > realize.
>> > > >
>> > > > Consider we have N queues, the number of vectors needed is 2*N + 2
>> > > > (#queue pairs + plus one config interrupt and control vq). We
>> > > > didn't
>> > > > check whether or not host support control vq because it was added
>> > > > unconditionally by qemu to avoid breaking legacy guests such as
>> > > > Minix.
>> > > >
>> > > > Reviewed-by: Philippe Mathieu-Daudé <[email protected]
>> > > > Reviewed-by: Stefano Garzarella <[email protected]>
>> > > > Reviewed-by: Stefan Hajnoczi <[email protected]>
>> > > > Signed-off-by: Jason Wang <[email protected]>
>> > >
>> > > Yes, devices designed according to the spec need to reserve an interrupt
>> > > vector for ctrlq. So, Michael, do we want to be compatible with buggy
>> > > devices?
>> > >
>> > > Thanks.
>> >
>> > These aren't buggy, the spec allows this. So don't fail, but
>> > I'm fine with using polling if not enough vectors.
>>
>> sharing with config interrupt is easier code-wise though, FWIW -
>> we don't need to maintain two code-paths.
>
>Yes, it works well - config change irq is used less before - and will not fail.
Please note I'm working on such fallback for admin queue. I would Like
to send the patchset by the end of this week. You can then use it easily
for cvq.
Something like:
/* the config->find_vqs() implementation */
int vp_find_vqs(struct virtio_device *vdev, unsigned int nvqs,
struct virtqueue *vqs[], vq_callback_t *callbacks[],
const char * const names[], const bool *ctx,
struct irq_affinity *desc)
{
int err;
/* Try MSI-X with one vector per queue. */
err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names,
VP_VQ_VECTOR_POLICY_EACH, ctx, desc);
if (!err)
return 0;
/* Fallback: MSI-X with one shared vector for config and
* slow path queues, one vector per queue for the rest. */
err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names,
VP_VQ_VECTOR_POLICY_SHARED_SLOW, ctx, desc);
if (!err)
return 0;
/* Fallback: MSI-X with one vector for config, one shared for queues. */
err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names,
VP_VQ_VECTOR_POLICY_SHARED, ctx, desc);
if (!err)
return 0;
/* Is there an interrupt? If not give up. */
if (!(to_vp_device(vdev)->pci_dev->irq))
return err;
/* Finally fall back to regular interrupts. */
return vp_find_vqs_intx(vdev, nvqs, vqs, callbacks, names, ctx);
}
>
>Thanks.
>
>>
>> > > >
>> > > > Thanks
>> > > >
>> > > > >
>> > > > > > > So no, you can not just do it unconditionally.
>> > > > > > >
>> > > > > > > The correct fix probably requires virtio core/API extensions.
>> > > > > >
>> > > > > > If the introduction of cvq irq causes interrupts to become shared,
>> > > > > > then
>> > > > > > ctrlq need to fall back to polling mode and keep the status quo.
>> > > > >
>> > > > > Having to path sounds a burden.
>> > > > >
>> > > > > >
>> > > > > > Thanks.
>> > > > > >
>> > > > >
>> > > > >
>> > > > > Thanks
>> > > > >
>> > > > > [1] https://www.linux-kvm.org/page/Multiqueue
>> > > > >
>> > > > > > >
>> > > > > > > --
>> > > > > > > MST
>> > > > > > >
>> > > > > >
>> > > >
>>
>
