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

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Date:   Fri, 8 Jun 2018 16:45:14 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Pierre Morel <pmorel@...ux.ibm.com>,
        Dong Jia Shi <bjsdjshi@...ux.ibm.com>,
        linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, qemu-s390x@...gnu.org,
        qemu-devel@...gnu.org
Subject: Re: [PATCH RFC 2/2] vfio-ccw: support for halt/clear subchannel

On Fri, 8 Jun 2018 15:13:28 +0200
Halil Pasic <pasic@...ux.ibm.com> wrote:

> 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).

It might make sense to keep this for ssch, maybe reuse it for hsch/csch,
and think about something else for other things we want to handle
(xsch, channel monitoring, the path handling stuff for which we already
had a prototype etc.) It's probably not practical to do radical surgery
on the existing code.

[Speaking of which: Is there any current effort on the path handling
things?]