On Thu, Dec 21, 2023 at 3:38 PM Dragos Tatulea <dtatu...@nvidia.com> wrote:
>
> On Thu, 2023-12-21 at 13:08 +0100, Eugenio Perez Martin wrote:
> > On Thu, Dec 21, 2023 at 12:52 PM Dragos Tatulea <dtatu...@nvidia.com> wrote:
> > >
> > > On Thu, 2023-12-21 at 08:46 +0100, Eugenio Perez Martin wrote:
> > > > On Thu, Dec 21, 2023 at 3:03 AM Jason Wang <jasow...@redhat.com> wrote:
> > > > >
> > > > > On Wed, Dec 20, 2023 at 9:32 PM Eugenio Perez Martin
> > > > > <epere...@redhat.com> wrote:
> > > > > >
> > > > > > On Wed, Dec 20, 2023 at 5:06 AM Jason Wang <jasow...@redhat.com>
> > > > > > wrote:
> > > > > > >
> > > > > > > On Wed, Dec 20, 2023 at 11:46 AM Jason Wang <jasow...@redhat.com>
> > > > > > > wrote:
> > > > > > > >
> > > > > > > > On Wed, Dec 20, 2023 at 2:09 AM Dragos Tatulea
> > > > > > > > <dtatu...@nvidia.com> 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.
> > > >
> > > In the current state, there is no resume. HW Virtqueues will just get
> > > re-created
> > > with the new address.
> > >
> >
> > Oh, then all of this is effectively transparent to the userspace
> > except for the time it takes?
> >
> Not quite: mlx5_vdpa_set_vq_address will save the vq address only on the SW vq
> representation. Only later will it will call into the FW to update the FW.
> Later
> means:
> - On DRIVER_OK state, when the VQs get created.
> - On .set_map when the VQs get re-created (before this series) / updated
> (after
> this series)
> - On .resume (after this series).
>
> So if the .set_vq_address is called when the VQ is in DRIVER_OK but not
> suspended those addresses will be set later for later.
>
Ouch, that is more in the line of my thoughts :(.
> > In that case you're right, we don't need feature flags. But I think it
> > would be great to also move the error return in case userspace tries
> > to modify vq parameters out of suspend state.
> >
> On the driver side or on the core side?
>
Core side.
It does not have to be part of this series, I meant it can be proposed
in a separate series and applied before the parent driver one.
> Thanks
> > Thanks!
> >
> >
> > > > 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.
> > > >
> > >
> >
>
