linux-kernel - Re: [PATCH vhost v4 02/15] vdpa: Add VHOST_BACKEND_F_CHANGEABLE_VQ_ADDR_IN

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Message-ID: <CAJaqyWeUHiZXMFkNBpinCsJAXojtPkGz+SjzUNDPx5W=qqON1w@mail.gmail.com>
Date: Thu, 21 Dec 2023 13:08:27 +0100
From: Eugenio Perez Martin <eperezma@...hat.com>
To: Dragos Tatulea <dtatulea@...dia.com>
Cc: "jasowang@...hat.com" <jasowang@...hat.com>, 
	"xuanzhuo@...ux.alibaba.com" <xuanzhuo@...ux.alibaba.com>, Parav Pandit <parav@...dia.com>, 
	"virtualization@...ts.linux-foundation.org" <virtualization@...ts.linux-foundation.org>, Gal Pressman <gal@...dia.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"si-wei.liu@...cle.com" <si-wei.liu@...cle.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"mst@...hat.com" <mst@...hat.com>, Saeed Mahameed <saeedm@...dia.com>, "leon@...nel.org" <leon@...nel.org>
Subject: Re: [PATCH vhost v4 02/15] vdpa: Add VHOST_BACKEND_F_CHANGEABLE_VQ_ADDR_IN_SUSPEND
 flag

On Thu, Dec 21, 2023 at 12:52 PM Dragos Tatulea <dtatulea@...dia.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 <jasowang@...hat.com> wrote:
> > >
> > > On Wed, Dec 20, 2023 at 9:32 PM Eugenio Perez Martin
> > > <eperezma@...hat.com> wrote:
> > > >
> > > > On Wed, Dec 20, 2023 at 5:06 AM Jason Wang <jasowang@...hat.com> wrote:
> > > > >
> > > > > On Wed, Dec 20, 2023 at 11:46 AM Jason Wang <jasowang@...hat.com> wrote:
> > > > > >
> > > > > > On Wed, Dec 20, 2023 at 2:09 AM Dragos Tatulea <dtatulea@...dia.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?

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 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.
> >
>