Re: [PATCH v2] Add device reset timeout field

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Oct 12, 2021 at 08:51:34AM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Monday, October 11, 2021 9:30 PM
> > 
> > On Mon, Oct 11, 2021 at 03:44:14PM +0000, Parav Pandit wrote:
> > > > > This is unlikely to work the reset is completed. Because a real
> > > > > device
> > > > implementing this would prefer to do this in fw for 1000 virtio
> > > > devices sitting on the physical card.
> > > > > And it is very much driven by such implementation at device devel.
> > > > > So it cannot update the counter value if reset is not completed for the
> > device.
> > > > >
> > > > > I think read only device reset timeout is most elegant option
> > > > > during device
> > > > initialization phase that eliminates infinite loop of today.
> > > >
> > > > Why can't a driver just go ahead and do a timeout regardless?
> > > o.k. lets consider this thought exercise. What is the timeout value that driver
> > will choose if device doesn't specify one?
> > > I explained in previous thread and you acked that actual fw based device
> > may take longer to initialize than pure sw implementation backend.
> > > In second example a pre-boot device can take even longer initialization time.
> > > Sriov VF device may initialize lot faster.
> > > Instead of driver having such transport, and device specific checks, (or some
> > very short or very long timeout), we propose, that let device mention such
> > timeout value.
> > 
> > Parav I think you are conflating reset with initialization time.
> > initialization is just for host boot which takes seconds anyway - but no,
> > minutes is not reasonable their, either.
> > reset affects guest boot. This needs to complete in milliseconds.
> >
> I cannot promise, but with newer generation devices usually functionality improves.
> Enforcing in milliseconds doesn't look practical for type of devices.
> Some of the block devices may need to establish TCP connections in the backend.
> It is more useful to wait for few more seconds to initialize device after power on the system, instead of giving up booting the server completely.
> For example, a nvme block device starts with a minimum timeout of 500msec.
> 
> Yes, I agree to your point that a device given to a guest VM will likely have very short reset time that should complete in milliseconds.
>  
> > This conflation is IMHO one of the problems with this proposal.
> 
> Device initialization consist of device reset from the spec section 3.1.1.

It does. But maybe we need to create a way for driver to distinguish
between the two. When under reset, use a driver supplied timeout.
When powering up, use a longer device supplied one.
migration is not a problem for baremetal so all's good from
that point of view. And power up seems irrelevant for
ccw/mmio since these are always within VMs. So it's a pci only thing.

-- 
MST