Re: [PATCH vhost v4 02/15] vdpa: Add VHOST_BACKEND_F_CHANGEABLE_VQ_ADDR_IN_SUSPEND flag

[Eugenio Perez Martin]

On Thu, Dec 21, 2023 at 3:03 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> On Wed, Dec 20, 2023 at 9:32 PM Eugenio Perez Martin
> <eperezma@xxxxxxxxxx> wrote:
> >
> > On Wed, Dec 20, 2023 at 5:06 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > >
> > > On Wed, Dec 20, 2023 at 11:46 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > > >
> > > > On Wed, Dec 20, 2023 at 2:09 AM Dragos Tatulea <dtatulea@xxxxxxxxxx> wrote:
> > > > >
> > > > > The virtio spec doesn't allow changing virtqueue addresses after
> > > > > DRIVER_OK. Some devices do support this operation when the device is
> > > > > suspended. The VHOST_BACKEND_F_CHANGEABLE_VQ_ADDR_IN_SUSPEND flag
> > > > > advertises this support as a backend features.
> > > >
> > > > There's an ongoing effort in virtio spec to introduce the suspend state.
> > > >
> > > > So I wonder if it's better to just allow such behaviour?
> > >
> > > Actually I mean, allow drivers to modify the parameters during suspend
> > > without a new feature.
> > >
> >
> > That would be ideal, but how do userland checks if it can suspend +
> > change properties + resume?
>
> As discussed, it looks to me the only device that supports suspend is
> simulator and it supports change properties.
>
> E.g:
>
> static int vdpasim_set_vq_address(struct vdpa_device *vdpa, u16 idx,
>                                   u64 desc_area, u64 driver_area,
>                                   u64 device_area)
> {
>         struct vdpasim *vdpasim = vdpa_to_sim(vdpa);
>         struct vdpasim_virtqueue *vq = &vdpasim->vqs[idx];
>
>         vq->desc_addr = desc_area;
>         vq->driver_addr = driver_area;
>         vq->device_addr = device_area;
>
>         return 0;
> }
>

So in the current kernel master it is valid to set a different vq
address while the device is suspended in vdpa_sim. But it is not valid
in mlx5, as the FW will not be updated in resume (Dragos, please
correct me if I'm wrong). Both of them return success.

How can we know in the destination QEMU if it is valid to suspend &
set address? Should we handle this as a bugfix and backport the
change?

> >
> > The only way that comes to my mind is to make sure all parents return
> > error if userland tries to do it, and then fallback in userland.
>
> Yes.
>
> > I'm
> > ok with that, but I'm not sure if the current master & previous kernel
> > has a coherent behavior. Do they return error? Or return success
> > without changing address / vq state?
>
> We probably don't need to worry too much here, as e.g set_vq_address
> could fail even without suspend (just at uAPI level).
>

I don't get this, sorry. I rephrased my point with an example earlier
in the mail.