Re: [RFC] vduse config write support

[Srivatsa Vaddagiri <quic_svaddagi@quicinc.com>]

* Yongji Xie <xieyongji@bytedance.com> [2024-07-26 10:37:40]:

> On Wed, Jul 24, 2024 at 11:38???AM Srivatsa Vaddagiri
> <quic_svaddagi@quicinc.com> wrote:
> >
> > Currently vduse does not seem to support configuration space writes
> > (vduse_vdpa_set_config does nothing). Is there any plan to lift that
> > limitation? I am aware of the discussions that took place here:
> >
> 
> The problem is that current virtio code can't allow the failure of config write.

Ok got it.

> > We will however likely need vduse to support configuration writes (guest VM
> > updating configuration space, for ex: writing to 'events_clear' field in case of
> > virtio-gpu). Would vduse maintainers be willing to accept config_write support
> > for select devices/features (as long as the writes don't violate any safety
> > concerns we may have)?
> >
> 
> It would be easier to support it if the config write just triggers an
> async operation on the device side, e.g. a doorbell. That means we
> can't ensure that any required internal actions on the device side
> triggered by the config write have been completed after the driver
> gets a successful return. But I'm not sure if this is your case.

Yes posted write should be fine, as long as guest issues a read as a fence after
that, which needs to be a sync point. As discussed in my earlier reply, we can
explore injecting surprise removal event into guest where the VDUSE daemon does
not respond within a timeout.

Thanks
vatsa