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

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Message-Id: <d68340a2-bb1b-cef9-e394-f0d55983c0bb@linux.ibm.com>
Date:   Fri, 8 Jun 2018 15:13:28 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>,
        Pierre Morel <pmorel@...ux.ibm.com>
Cc:     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 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