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From: Eugenio Perez Martin <eperezma@redhat.com>
Date: Thu, 21 Dec 2023 13:08:27 +0100
Message-ID: <CAJaqyWeUHiZXMFkNBpinCsJAXojtPkGz+SjzUNDPx5W=qqON1w@mail.gmail.com>
Subject: Re: [PATCH vhost v4 02/15] vdpa: Add VHOST_BACKEND_F_CHANGEABLE_VQ_ADDR_IN_SUSPEND
 flag
To: Dragos Tatulea <dtatulea@nvidia.com>
Cc: "jasowang@redhat.com" <jasowang@redhat.com>, 
	"xuanzhuo@linux.alibaba.com" <xuanzhuo@linux.alibaba.com>, Parav Pandit <parav@nvidia.com>, 
	"virtualization@lists.linux-foundation.org" <virtualization@lists.linux-foundation.org>, Gal Pressman <gal@nvidia.com>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, 
	"si-wei.liu@oracle.com" <si-wei.liu@oracle.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, 
	"mst@redhat.com" <mst@redhat.com>, Saeed Mahameed <saeedm@nvidia.com>, "leon@kernel.org" <leon@kernel.org>
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On Thu, Dec 21, 2023 at 12:52=E2=80=AFPM Dragos Tatulea <dtatulea@nvidia.co=
m> wrote:
>
> On Thu, 2023-12-21 at 08:46 +0100, Eugenio Perez Martin wrote:
> > On Thu, Dec 21, 2023 at 3:03=E2=80=AFAM Jason Wang <jasowang@redhat.com=
> wrote:
> > >
> > > On Wed, Dec 20, 2023 at 9:32=E2=80=AFPM Eugenio Perez Martin
> > > <eperezma@redhat.com> wrote:
> > > >
> > > > On Wed, Dec 20, 2023 at 5:06=E2=80=AFAM Jason Wang <jasowang@redhat=
.com> wrote:
> > > > >
> > > > > On Wed, Dec 20, 2023 at 11:46=E2=80=AFAM Jason Wang <jasowang@red=
hat.com> wrote:
> > > > > >
> > > > > > On Wed, Dec 20, 2023 at 2:09=E2=80=AFAM Dragos Tatulea <dtatule=
a@nvidia.com> wrote:
> > > > > > >
> > > > > > > The virtio spec doesn't allow changing virtqueue addresses af=
ter
> > > > > > > DRIVER_OK. Some devices do support this operation when the de=
vice 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 suspe=
nd state.
> > > > > >
> > > > > > So I wonder if it's better to just allow such behaviour?
> > > > >
> > > > > Actually I mean, allow drivers to modify the parameters during su=
spend
> > > > > 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 =3D vdpa_to_sim(vdpa);
> > >         struct vdpasim_virtqueue *vq =3D &vdpasim->vqs[idx];
> > >
> > >         vq->desc_addr =3D desc_area;
> > >         vq->driver_addr =3D driver_area;
> > >         vq->device_addr =3D 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?

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.

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 retu=
rn
> > > > 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 ker=
nel
> > > > 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.
> >
>