[Virtio-fs] virtio-fs on zLinux (s390x)

[Cornelia Huck]

On Thu, 12 Dec 2019 18:59:00 +0000
"Dr. David Alan Gilbert" <dgilbert at redhat.com> wrote:

> * Gong Su (gongsu at us.ibm.com) wrote:
> > 
> > 
> > Hi Stefan, I'm resending my questions to the list per your request.
> > 
> > Basically, I'm trying to see if I can get virtio-fs to work on zLinux.
> > Following the instructions on https://virtio-fs.gitlab.io/howto-qemu.html I
> > was able to compile all the pieces but when I tried to run qemu I realized
> > that it needs -numa option which isn't supported on s390x yet.
> > 
> > I have talked to Christian Borntraeger (whom I worked with on other
> > projects before) and he indicated that he had a plan to add -numa support
> > for s390x but currently is busy on other things.
> > 
> > So I'm trying to see if there are other ways for me to go forward without
> > having to wait for Christian. I see two options:
> > 
> > 1. Merge virtiofsd into qemu itself. In the "Interchangeability" section of
> > the virtio-fs design page (https://virtio-fs.gitlab.io/design.html), it
> > mentioned that "It is also possible to forego vhost-user and emulate the
> > virtio-fs device directly inside the hypervisor, although this may result
> > in poor isolation and security." Since each guest typically gets its own
> > qemu process, maybe this is acceptable for some use cases. In exchange, you
> > get the benefit of not having to carve out a chunk of memory from /dev/shm
> > (and maybe simpler communication between virtiofsd and qemu).
> > 2. Use something other than -numa to share memory between virtiofsd and
> > qemu that's supported on zLinux. I'm not entirely familiar with qemu so I
> > have no idea whether this is possible or not.  
> 
> Hi,
>   My understanding is that Igor Mammedov (imammedo at redhat.com) has plans
> to allow 'shared' to be set on main memory, so we wouldn't need the numa
> code.

Yes, that was my understanding as well.

Obviously, nobody has actually tested virtio-fs on s390x so far :)
Wiring it up for virtio-ccw looks straightforward, though, and I did
not spot any obvious problems when looking at the code a while back.

One thing that needs more thought is DAX support in the future (i.e.
where to put the shared regions for ccw etc.) There had been some
discussion on that topic in the past, but I currently don't have any
spare cycles for continuing there. That said, more s390x folks looking
at this would certainly be a good idea :)