Subject: Re: [PATCH vhost 20/23] vdpa/mlx5: Pre-create hardware VQs at vdpa .dev_add time

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 8 Jul 2024 07:25:42 -0400

On Mon, Jul 08, 2024 at 11:17:06AM +0000, Dragos Tatulea wrote:
> On Mon, 2024-07-08 at 07:11 -0400, Michael S. Tsirkin wrote:
> > On Mon, Jul 08, 2024 at 11:01:39AM +0000, Dragos Tatulea wrote:
> > > On Wed, 2024-07-03 at 18:01 +0200, Eugenio Perez Martin wrote:
> > > > On Wed, Jun 26, 2024 at 11:27 AM Dragos Tatulea <[email-masked]> wrote:
> > > > > 
> > > > > On Wed, 2024-06-19 at 17:54 +0200, Eugenio Perez Martin wrote:
> > > > > > On Mon, Jun 17, 2024 at 5:09 PM Dragos Tatulea <[email-masked]> wrote:
> > > > > > > 
> > > > > > > Currently, hardware VQs are created right when the vdpa device gets into
> > > > > > > DRIVER_OK state. That is easier because most of the VQ state is known by
> > > > > > > then.
> > > > > > > 
> > > > > > > This patch switches to creating all VQs and their associated resources
> > > > > > > at device creation time. The motivation is to reduce the vdpa device
> > > > > > > live migration downtime by moving the expensive operation of creating
> > > > > > > all the hardware VQs and their associated resources out of downtime on
> > > > > > > the destination VM.
> > > > > > > 
> > > > > > > The VQs are now created in a blank state. The VQ configuration will
> > > > > > > happen later, on DRIVER_OK. Then the configuration will be applied when
> > > > > > > the VQs are moved to the Ready state.
> > > > > > > 
> > > > > > > When .set_vq_ready() is called on a VQ before DRIVER_OK, special care is
> > > > > > > needed: now that the VQ is already created a resume_vq() will be
> > > > > > > triggered too early when no mr has been configured yet. Skip calling
> > > > > > > resume_vq() in this case, let it be handled during DRIVER_OK.
> > > > > > > 
> > > > > > > For virtio-vdpa, the device configuration is done earlier during
> > > > > > > .vdpa_dev_add() by vdpa_register_device(). Avoid calling
> > > > > > > setup_vq_resources() a second time in that case.
> > > > > > > 
> > > > > > 
> > > > > > I guess this happens if virtio_vdpa is already loaded, but I cannot
> > > > > > see how this is different here. Apart from the IOTLB, what else does
> > > > > > it change from the mlx5_vdpa POV?
> > > > > > 
> > > > > I don't understand your question, could you rephrase or provide more context
> > > > > please?
> > > > > 
> > > > 
> > > > My main point is that the vdpa parent driver should not be able to
> > > > tell the difference between vhost_vdpa and virtio_vdpa. The only
> > > > difference I can think of is because of the vhost IOTLB handling.
> > > > 
> > > > Do you also observe this behavior if you add the device with "vdpa
> > > > add" without the virtio_vdpa module loaded, and then modprobe
> > > > virtio_vdpa?
> > > > 
> > > Aah, now I understand what you mean. Indeed in my tests I was loading the
> > > virtio_vdpa module before adding the device. When doing it the other way around
> > > the device doesn't get configured during probe.
> > >  
> > > 
> > > > At least the comment should be something in the line of "If we have
> > > > all the information to initialize the device, pre-warm it here" or
> > > > similar.
> > > Makes sense. I will send a v3 with the commit + comment message update.
> > 
> > 
> > Is commit update the only change then?
> I was planning to drop the paragraph in the commit message (it is confusing) and
> edit the comment below (scroll down to see which).
> 
> Let me know if I should send the v3 or not. I have it prepared.

You can do this but pls document that the only change is in commit log.

> > 
> > > > 
> > > > > Thanks,
> > > > > Dragos
> > > > > 
> > > > > > > Signed-off-by: Dragos Tatulea <[email-masked]>
> > > > > > > Reviewed-by: Cosmin Ratiu <[email-masked]>
> > > > > > > ---
> > > > > > >  drivers/vdpa/mlx5/net/mlx5_vnet.c | 37 ++++++++++++++++++++++++++++++++-----
> > > > > > >  1 file changed, 32 insertions(+), 5 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > index 249b5afbe34a..b2836fd3d1dd 100644
> > > > > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > > > > @@ -2444,7 +2444,7 @@ static void mlx5_vdpa_set_vq_ready(struct vdpa_device *vdev, u16 idx, bool ready
> > > > > > >         mvq = &ndev->vqs[idx];
> > > > > > >         if (!ready) {
> > > > > > >                 suspend_vq(ndev, mvq);
> > > > > > > -       } else {
> > > > > > > +       } else if (mvdev->status & VIRTIO_CONFIG_S_DRIVER_OK) {
> > > > > > >                 if (resume_vq(ndev, mvq))
> > > > > > >                         ready = false;
> > > > > > >         }
> > > > > > > @@ -3078,10 +3078,18 @@ static void mlx5_vdpa_set_status(struct vdpa_device *vdev, u8 status)
> > > > > > >                                 goto err_setup;
> > > > > > >                         }
> > > > > > >                         register_link_notifier(ndev);
> > > > > > > -                       err = setup_vq_resources(ndev, true);
> > > > > > > -                       if (err) {
> > > > > > > -                               mlx5_vdpa_warn(mvdev, "failed to setup driver\n");
> > > > > > > -                               goto err_driver;
> > > > > > > +                       if (ndev->setup) {
> > > > > > > +                               err = resume_vqs(ndev);
> > > > > > > +                               if (err) {
> > > > > > > +                                       mlx5_vdpa_warn(mvdev, "failed to resume VQs\n");
> > > > > > > +                                       goto err_driver;
> > > > > > > +                               }
> > > > > > > +                       } else {
> > > > > > > +                               err = setup_vq_resources(ndev, true);
> > > > > > > +                               if (err) {
> > > > > > > +                                       mlx5_vdpa_warn(mvdev, "failed to setup driver\n");
> > > > > > > +                                       goto err_driver;
> > > > > > > +                               }
> > > > > > >                         }
> > > > > > >                 } else {
> > > > > > >                         mlx5_vdpa_warn(mvdev, "did not expect DRIVER_OK to be cleared\n");
> > > > > > > @@ -3142,6 +3150,7 @@ static int mlx5_vdpa_compat_reset(struct vdpa_device *vdev, u32 flags)
> > > > > > >                 if (mlx5_vdpa_create_dma_mr(mvdev))
> > > > > > >                         mlx5_vdpa_warn(mvdev, "create MR failed\n");
> > > > > > >         }
> > > > > > > +       setup_vq_resources(ndev, false);
> > > > > > >         up_write(&ndev->reslock);
> > > > > > > 
> > > > > > >         return 0;
> > > > > > > @@ -3836,8 +3845,21 @@ static int mlx5_vdpa_dev_add(struct vdpa_mgmt_dev *v_mdev, const char *name,
> > > > > > >                 goto err_reg;
> > > > > > > 
> > > > > > >         mgtdev->ndev = ndev;
> > > > > > > +
> > > > > > > +       /* For virtio-vdpa, the device was set up during device register. */
> > > > > > > +       if (ndev->setup)
> > > > > > > +               return 0;
> > > > > > > +
> This comment updated to:
> 
> /* The VQs might have been pre-created during device register.
>  * This happens when virtio_vdpa is loaded before the vdpa device is added.
>  */
> 
> 
> > > > > > > +       down_write(&ndev->reslock);
> > > > > > > +       err = setup_vq_resources(ndev, false);
> > > > > > > +       up_write(&ndev->reslock);
> > > > > > > +       if (err)
> > > > > > > +               goto err_setup_vq_res;
> > > > > > > +
> > > > > > >         return 0;
> > > > > > > 
> > > > > > > +err_setup_vq_res:
> > > > > > > +       _vdpa_unregister_device(&mvdev->vdev);
> > > > > > >  err_reg:
> > > > > > >         destroy_workqueue(mvdev->wq);
> > > > > > >  err_res2:
> > > > > > > @@ -3863,6 +3885,11 @@ static void mlx5_vdpa_dev_del(struct vdpa_mgmt_dev *v_mdev, struct vdpa_device *
> > > > > > > 
> > > > > > >         unregister_link_notifier(ndev);
> > > > > > >         _vdpa_unregister_device(dev);
> > > > > > > +
> > > > > > > +       down_write(&ndev->reslock);
> > > > > > > +       teardown_vq_resources(ndev);
> > > > > > > +       up_write(&ndev->reslock);
> > > > > > > +
> > > > > > >         wq = mvdev->wq;
> > > > > > >         mvdev->wq = NULL;
> > > > > > >         destroy_workqueue(wq);
> > > > > > > 
> > > > > > > --
> > > > > > > 2.45.1
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> Thanks,
> Dragos
>