Re: [qemu-s390x] [PATCH RFC 2/2] vfio-ccw: support for halt/clear subchannel

[Halil Pasic]

On 06/08/2018 02:20 PM, Cornelia Huck wrote:
> > > My proposal is to do the same
> > > copying to scsw(r) again, which would mean we get a request with both
> > > the halt and the start bit set. The vfio code now needs to do a hsch
> > > (instead of a ssch). The real channel subsystem should figure this out,
> > > as we can't reliably check whether the start function has concluded
> > > already (there's always a race window).
> > This I do not agree scsw(r) is part of the driver.
> > The interface here is not a device interface anymore but a driver
> > interface.
> > SCSW is a status, it is at its place in QEMU device interface with the
> > guest
> > but here pwrite() sends a command.
> Hm, I rather consider that "we write a status, and the backend figures
> out what to do based on that status".

The status of what? Kind of a target status?

I think this approach is the source of lots of complications. For instance
take xsch. How are we supposed to react to a guest xsch (in QEMU and
in the kernel module)? My guess is that the right thing to do is to issue
an xsch in the vfio-ccw kernel module on the passed through subchannel.
But there is no bit in fctl for cancel.

Bottom line is: I'm not happy with the current design but I'm not sure
if it's practical to do something about it (i.e. change it radically).

Regards,
Halil