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

From: Christian Borntraeger
Subject: Re: [Qemu-devel] Questions about usability mess that caused by differentiating address based on devices types
Date: Tue, 14 Nov 2017 12:17:46 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/14/2017 11:50 AM, Cornelia Huck wrote:
> 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...)

Yes and I think we can assume that MCSS-E will not come in the foreseable 
> 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 :)

I never looked into the details, but my expectation for the squash thing
was that it actually allows to add real devices into the virtual channel
subsystem. So maybe we should really do 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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