On Fri, Jul 21, 2023 at 04:16:07PM +0200, Hanna Czenczek wrote:
> On 20.07.23 18:03, Stefan Hajnoczi wrote:
> > On Wed, Jul 19, 2023 at 04:27:58PM +0200, Hanna Czenczek wrote:
> > > On 19.07.23 16:11, Hanna Czenczek wrote:
> > > > On 18.07.23 17:10, Stefan Hajnoczi wrote:
> > > > > On Tue, Jul 11, 2023 at 05:52:28PM +0200, Hanna Czenczek wrote:
> > > > > > The only user of vhost_user_reset_status() is vhost_dev_stop(),
> > > > > > which
> > > > > > only uses it as a fall-back to stop the back-end if it does not
> > > > > > support
> > > > > > SUSPEND. However, vhost-user's implementation is a no-op unless the
> > > > > > back-end supports SET_STATUS.
> > > > > >
> > > > > > vhost-vdpa's implementation instead just calls
> > > > > > vhost_vdpa_reset_device(), implying that it's OK to fully reset the
> > > > > > device if SET_STATUS is not supported.
> > > > > >
> > > > > > To be fair, vhost_vdpa_reset_device() does nothing but to set
> > > > > > the status
> > > > > > to zero. However, that may well be because vhost-vdpa has no method
> > > > > > besides this to reset a device. In contrast, vhost-user has
> > > > > > RESET_DEVICE and a RESET_OWNER, which can be used instead.
> > > > > >
> > > > > > While it is not entirely clear from documentation or git logs, from
> > > > > > discussions and the order of vhost-user protocol features, it
> > > > > > appears to
> > > > > > me as if RESET_OWNER originally had no real meaning for vhost-user,
> > > > > > and
> > > > > > was thus used to signal a device reset to the back-end. Then,
> > > > > > RESET_DEVICE was introduced, to have a well-defined dedicated reset
> > > > > > command. Finally, vhost-user received full STATUS support,
> > > > > > including
> > > > > > SET_STATUS, so setting the device status to 0 is now the preferred
> > > > > > way
> > > > > > of resetting a device. Still, RESET_DEVICE and RESET_OWNER should
> > > > > > remain valid as fall-backs.
> > > > > >
> > > > > > Therefore, have vhost_user_reset_status() fall back to
> > > > > > vhost_user_reset_device() if the back-end has no STATUS support.
> > > > > >
> > > > > > Signed-off-by: Hanna Czenczek <hre...@redhat.com>
> > > > > > ---
> > > > > > hw/virtio/vhost-user.c | 2 ++
> > > > > > 1 file changed, 2 insertions(+)
> > > > > >
> > > > > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > > > > > index 4507de5a92..53a881ec2a 100644
> > > > > > --- a/hw/virtio/vhost-user.c
> > > > > > +++ b/hw/virtio/vhost-user.c
> > > > > > @@ -2833,6 +2833,8 @@ static void vhost_user_reset_status(struct
> > > > > > vhost_dev *dev)
> > > > > > if (virtio_has_feature(dev->protocol_features,
> > > > > > VHOST_USER_PROTOCOL_F_STATUS)) {
> > > > > > vhost_user_set_status(dev, 0);
> > > > > > + } else {
> > > > > > + vhost_user_reset_device(dev);
> > > > > > }
> > > > > > }
> > > > > Did you check whether DPDK treats setting the status to 0 as
> > > > > equivalent
> > > > > to RESET_DEVICE?
> > > > If it doesn’t, what’s even the point of using reset_status?
> > > Sorry, I’m being unclear, and I think this may be important because it
> > > ties
> > > into the question from patch 1, what qemu is even trying to do by running
> > > SET_STATUS(0) vhost_dev_stop(), so here’s what gave me the impression that
> > > SET_STATUS(0) and RESET_DEVICE should be equivalent:
> > >
> > > vhost-vdpa.c runs SET_STATUS(0) in a function called
> > > vhost_vdpa_reset_device(). This is one thing that gave me the impression
> > > that this is about an actual full reset.
> > >
> > > Another is the whole discussion that we’ve had. vhost_dev_stop() does not
> > > call a `vhost_reset_device()` function, it calls `vhost_reset_status()`.
> > > Still, we were always talking about resetting the device.
> > There is some hacky stuff with struct vhost_dev's vq_index_end and
> > multi-queue devices. I think it's because multi-queue vhost-net device
> > consist of many vhost_devs and NetClientStates, so certain vhost
> > operations are skipped unless this is the "first" or "last" vhost_dev
> > from a large aggregate vhost-net device. That might be responsible for
> > part of the weirdness.
> >
> > > It doesn’t make sense to me that vDPA would provide no function to fully
> > > reset a device, while vhost-user does. Being able to reset a device
> > > sounds
> > > vital to me. This also gave me the impression that SET_STATUS(0) on vDPA
> > > at
> > > least is functionally equivalent to a full device reset.
> > >
> > >
> > > Maybe SET_STATUS(0) does mean a full device reset on vDPA, but not on
> > > vhost-user. That would be a real shame, so I assumed this would not be
> > > the
> > > case; that SET_STATUS(0) does the same thing on both protocols.
> > Yes, exactly. It has the real VIRTIO spec meaning in vDPA. In vhost-user
> > it's currently only used by DPDK as a hint for when device
> > initialization is complete:
> > https://github.com/DPDK/dpdk/commit/41d201804c4c44738168e2d247d3b1780845faa1
>
> FWIW, now the code is a bit different.
> https://github.com/DPDK/dpdk/commit/671cc679a5fcd26705bb20ddc13b93e665719054
> has added a RESET interpretation for the status field, i.e. when it is 0.
> It doesn’t do anything, but at least DPDK seems to agree that SET_STATUS(0)
> is a reset.That patch adds diagnostics but does not perform any action for SET_STATUS 0. DPDK's vhost_user_reset_owner() is still the only place where the device is actually reset. QEMU cannot switch to just SET_STATUS 0, it still needs to send RESET_DEVICE/RESET_OWNER. > > > > The virtio specification says “Writing 0 into this field resets the > > > device.” > > > about the device_status field. > > > > > > This also makes sense, because the device_status field is basically used > > > to > > > tell the device that a driver has taken control. If reset, this indicates > > > the driver has given up control, and to me this is a point where a device > > > should fully reset itself. > > > > > > So all in all, I can’t see the rationale why any implementation that > > > supports SET_STATUS would decide to treat SET_STATUS(0) not as equivalent > > > or > > > a superset of RESET_DEVICE. I may be wrong, and this might explain a > > > whole > > > deal about what kind of background operations we hope to stop with > > > SET_STATUS(0). > > I would like vhost-user devices to implement SET_STATUS according to the > > VIRTIO specification in the future and they can do that. But I think > > front-ends should continue sending RESET_DEVICE in order to support old > > devices. > > Well, yes, exactly. That is what I meant to address with this patch, > vhost-user right now does not send RESET_DEVICE in its vhost_reset_status > implementation, so the front-end will not fall back to RESET_DEVICE when it > apparently does intend to reset the device[1]. We do arguably have > vhost_reset_device, too, but for vDPA that is just a SET_STATUS(0) (there is > no RESET_DEVICE on vDPA), and it’s also only called by vhost-user-scsi. > > So this also begs the question why we even do have vhost_reset_status and > vhost_reset_device as two separate things. The commit introducing > vhost_reset_status (c3716f260bf) doesn’t say. Maybe the intention was that > vhost_reset_device would leave the status at 0, while vhost_reset_status > would return it to ACKNOWLEDGE | DRIVER, as done by the introducing commit, > but that comes back to patch 5 in this series – we don’t need to have > ACKNOWLEDGE | DRIVER set after vhost_dev_stop(), so we don’t need > vhost_reset_status to set those flags. They should be set in > vhost_dev_start(). > > [1] This is assuming that SET_STATUS(0) is intended to reset the device, but > it sounds like you agree on that. I don't know the answers, but I think it's safe to go ahead with a SET_STATUS sequence that follows the VIRTIO spec, plus a VHOST_USER_RESET_DEVICE/VHOST_USER_RESET_OWNER. Stefan
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