On Wed, Sep 27, 2023 at 10:32:14AM +0200, Hanna Czenczek wrote: > On 26.09.23 22:10, Stefan Hajnoczi wrote: > > Hi Hanna, > > I was thinking about how this could work without SUSPEND/RESUME. What > > do you think of the following? > > > > 1. The front-end sends VHOST_USER_RESET_DEVICE (or > > VHOST_USER_RESET_OWNER, when necessary) when the guest driver resets > > the device but not on vhost_dev_start()/vhost_dev_stop(). > > This is half the work of SUSPEND/RESUME. It isn’t easy to do.
I sent a patch series to bring VHOST_USER_RESET_DEVICE to all vhost-user devices: https://lore.kernel.org/qemu-devel/20230927192737.528280-1-stefa...@redhat.com/T/#t > > > 2. Suspend the device when all virtqueues are stopped via > > VHOST_USER_GET_VRING_BASE. Resume the device after at least one > > virtqueue is started and enabled. > > 3. Ignore VHOST_USER_SET_STATUS. > > > > Reset would work. The device would suspend and resume without losing > > state. Existing vhost-user backends already behave like this in > > practice (they often don't implement RESET_DEVICE). > > I don’t understand the point, though. Today, reset in practice is a no-op > anyway, precisely because we only send SET_STATUS 0, don’t fall back to > RESET_OWNER/RESET_DEVICE, and no back-end implements SET_STATUS 0 as a > reset. By sending RESET_* in case of a guest-initiated reset and nothing in > case of stop/cont, we effectively don’t change anything about the latter > (which is what SUSPEND/RESUME would be for), but only fix the former case. > While I agree that it’s wrong that we don’t really reset the back-end in > case of a guest-initiated reset, this is the first time in this whole > discussion that that part has been presented as a problem that needs fixing > now. > > So the proposal effectively changes nothing for the vhost_dev_stop()/start() > case where we’d want to make use of SUSPEND/RESUME, but only for the case > where we would not use it. We discussed this on a call today. 2 & 3 are additions to the spec that Hanna has agreed to work on. Stefan
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