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Re: [Qemu-devel] [RFC PATCH 1/1] s390x/css: unresrict cssids

From: Christian Borntraeger
Subject: Re: [Qemu-devel] [RFC PATCH 1/1] s390x/css: unresrict cssids
Date: Wed, 29 Nov 2017 19:51:23 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/28/2017 03:45 PM, Cornelia Huck wrote:
> On Tue, 28 Nov 2017 15:17:49 +0100
> Christian Borntraeger <address@hidden> wrote:
>> On 11/28/2017 03:01 PM, Cornelia Huck wrote:
>>> On Tue, 28 Nov 2017 14:25:08 +0100
>>> Christian Borntraeger <address@hidden> wrote:
>>>> What I want now is to enable vfio-ccw for libvirt and Linux guests and for 
>>>> that
>>>> we need to lift the restriction of having vfio not in FE.  
>>> This, however, worries me. I don't really consider vfio-ccw ready for
>>> prime time yet. We still need to figure out path handling at the very
>>> least. And I'm still not sure that our non-handling of halt/clear won't
>>> cause major issues with e.g. a storage server error recovery.
>>> Can we flag vfio-ccw as experimental or such in libvirt?  
>> We should have flagged vfio-ccw experimental in QEMU then.
>> e.g. by using x-vfio-ccw or whatever.I dont think we can do that
>> after the facts.
> Yes, we should have done that... can't fix that now, unfortunately.
>> I am not that deep into vfio-ccw, but I was under the impression that
>> the missing features should not affect the vfio interface that is
>> created by libvirt. After all this should just be a "-device vfio-ccw,....."
>> statement and some setup steps beforehand (unbind + setup of vfio-ccw)
> The halt/clear stuff is highly unlikely to influence the configuration
> interface. I'm still not too clear about path handling, though. Will we
> need different modes (hypervisor managed vs. full passthrough? probably
> only passthrough)? What about pgid handling - do we need some kind of
> pgid manager? (Keep in mind that you get to set the pgid once. This has
> implications for e.g. reserve/release.)
> Also, what about devices other than ECKD DASD? Has there been any
> testing? Tapes should be interesting; the other interesting ccw devices
> are QDIO-based and I'm not sure we want to spend anything on supporting
> those.

DASD is probably the most important thing today, QDIO might never happen
(or very late).
See my proposal below.

> The interface is probably fine, but I'd like to get an idea about the
> path stuff (is the attachment spec that contains the pgid stuff
> publicly available, btw.?)
>> If your concern is the serviceability I think it would be valid for a RHEL
>> KVM to disable vfio-ccw in the kernel. Maybe we could even provide a config
>> for QEMU?
> It's fine just to turn off vfio-ccw in the kernel.
>> PS: I learned from the PCI and CCW experience that I do not want
>> to upstream kernel/qemu code unless I have a working libvirt code that
>> shows that the thing will work.
> Yes, I understand the wish to verify the interface, and I think it's a
> good idea. What I'm worried about is that this might be the precursor
> to a push to support it, and I don't think vfio-ccw is ready for that
> yet.
FWIW, libvirt should not care if a device works in all cases or not because 
libvirt versions should work with all kind of QEMU/kernel combinations. 
Fencing in libvirt that the kernel part is not up to the task is making
me feel the same way as you when you see  the css-unrestricted property
at a device ;-)
Having said that, I think that having vfio-ccw support has a value (and it
actually works for an unnamed test infrastructure). In addition to that I 
am much more likely to actually test this continuously if I have libvirt 

So what about the following: 

1. We will implement the libvirt support with either a or b:
a: if we find a solution to our "where to put the property" dispute use that to 
if we can add vfio-ccw
b if not: just provide a patch that lifts the restriction without any property.
in libvirt blindy assume  that free assignment will work. old QEMUs will 
complain at
startup and  libvirt will print the QEMU error. This is similar to other 
where libvirt cannot fully bprobe  if the QEMU will work or not. (not having 
support in the kernel will certainly allow libvirt to reject this upfront)

2. at the same time we mark vfio-ccw experimental in the kernel to make clear
 that this is still work in progress

3. (optional) we could even whitelist devices that work in a reasonable fashion 
do not whitelist qdio but whitelist DASD)

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