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Re: [Qemu-devel] Questions about usability mess that caused by different

From: Cornelia Huck
Subject: Re: [Qemu-devel] Questions about usability mess that caused by differentiating address based on devices types
Date: Tue, 14 Nov 2017 11:50:14 +0100

On Tue, 14 Nov 2017 16:25:47 +0800
Dong Jia Shi <address@hidden> wrote:

> Dear Conny,
> Good day!
> Just now, our Libvirt folks pointed out a "usability mess" for the
> design of differentiating address based on devices classes (real |
> virtual). The complaints are mainly about the "s390-squash-mcss"
> property and restrictions to define virtual device in the special 0xFE
> css, and define real devices in non-0xFE.
> We have some discussions internally, but failed to get it cleared. As we
> think this is about the architecture, so hereby, I as a representative,
> forward our arguments and questions here to ask you for help:
> 1. What benifit do we get to put virtual devices in css 0xFE?

Some background here (for the benefit of innocent bystanders):

In the past, I had been involved with some cases where Linux guests
under z/VM died after a customer followed the recommended procedure to
vary off a path before applying service. That path was supposed to be a
path to a disk; unfortunately, z/VM had mapped all kinds of virtual
paths to it, including the only path to the console device. Oops.

With that in mind, we wanted to make sure that qemu would not be
susceptible to the same problem; IOW, we wanted to make sure that
chpids etc. were not mashed together for devices that did not have
anything to do with each other. At one point in time, the idea came up
to use a reserved css for virtio devices, which was deemed an elegant
solution as 'real' devices were still something far in the future. (And
I was under the delusion that we would have MCSS-E support in Linux by
then; that has not happened...)

So the basic idea of css 0xfe is: Maintain a clear separation between
devices emulated by qemu and pass-through devices (a more divisive
separation than by simply separating chpids).

> 2. Since we could accept squashing virtual devices into css 0, can we
> accept to not trading 0xFE as a special css?

Using css 0xfe seemed like a good idea; but as things worked out
differently in the meantime, it seems it causes more problems right now
than it avoids.

> So that we can remove the restrictions for the cssid validation for each
> type of device. Even we could drop the s390-squash-mcss, and just allow
> the user to define any device in any css.

Opening up the different csses for all devices might help, but we need
to be careful:
- We still want to keep the chpids separated. Probably not a problem
  right now.
- We need to be able to point to a default css, especially as there are
  no MCSS-E capable OSs around yet.
- You need to double check if there are further restrictions on the
  allowed css ids. (I know that 0xff is reserved for special usage as
  well; but I can't find out more.)
- Backwards compatibility and migration: We certainly don't want old
  setups to break, and compat machines need to force the old scheme.

All best tested out via a prototype :)

> 3. If we have to keep the squash property, then when squashing, it's
> somewhat like "I don't care for the cssid", so is it possible for us to
> not check the cssid in the device devno?
> Libvirt would be benifited with this when automatically generating the
> addresses.

I think we still need to keep the squashing around for compatibility,
but we may be able to give it the chop for something like 3.0.

(And we probably need to keep the existing restrictions in compat mode.)

> 4. Error message for devno conflict is not helpful. For the following
> case:
>   -M s390-ccw-virtio,s390-squash-mcss=on \
>   -drive 
> file=/dev/disk/by-path/ccw-0.0.3f3e,if=none,id=drive-virtio-disk1,format=raw \
>   -device 
> virtio-blk-ccw,devno=fe.0.2222,scsi=off,drive=drive-virtio-disk1,id=virtio-disk1,bootindex=0
>  \
>   -device 
> vfio-ccw,devno=0.0.2222,sysfsdev=/sys/devices/css0/0.0.013f/6dfd3ec5-e8b3-4e18-a6fe-57bc9eceb920
>  \
> We get this error message:
> qemu-system-s390x: -device 
> vfio-ccw,devno=0.0.2222,sysfsdev=/sys/devices/css0/0.0.013f/6dfd3ec5-e8b3-4e18-a6fe-57bc9eceb920:
>  Device 0.0.2222 already exists
> By checking 0.0.2222 from the cmd line, users can not find out the root
> cause - squashing, easily. So, if we have to keep the squash property,
> we could improve this message by adding a hint.

That's probably a change that can be done quickly, without any compat
implications, right?

> To sum up, we got the feeling that, this mess is not only for Libvirt
> but also for QEMU cmd line users. And we are wondering if there is some
> way to improve it.

Using css 0xfe seems to be an idea that turned out not to be as useful
as we hoped it would be. Maybe the right way forward is indeed to open
up the csses for all devices (although there might be a case for
putting non-virtual devices not into 0xfe by default and instead making
0 the default css).

Another thing: Should libvirt give its users enough rope to hang
themselves by allowing to create domains with devices all over the
channel subsystem images?

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