Re: [Xen-devel] [RFC Design Doc] Add vNVDIMM support for Xen

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 4 Feb 2016, Haozhong Zhang wrote:
> On 02/03/16 15:22, Stefano Stabellini wrote:
> > On Wed, 3 Feb 2016, George Dunlap wrote:
> > > On 03/02/16 12:02, Stefano Stabellini wrote:
> > > > On Wed, 3 Feb 2016, Haozhong Zhang wrote:
> > > >> Or, we can make a file system on /dev/pmem0, create files on it, set
> > > >> the owner of those files to xen-qemuuser-domid$domid, and then pass
> > > >> those files to QEMU. In this way, non-root QEMU should be able to
> > > >> mmap those files.
> > > >
> > > > Maybe that would work. Worth adding it to the design, I would like to
> > > > read more details on it.
> > > >
> > > > Also note that QEMU initially runs as root but drops privileges to
> > > > xen-qemuuser-domid$domid before the guest is started. Initially QEMU
> > > > *could* mmap /dev/pmem0 while is still running as root, but then it
> > > > wouldn't work for any devices that need to be mmap'ed at run time
> > > > (hotplug scenario).
> > >
> > > This is basically the same problem we have for a bunch of other things,
> > > right?  Having xl open a file and then pass it via qmp to qemu should
> > > work in theory, right?
> >
> > Is there one /dev/pmem? per assignable region?
> 
> Yes.
> 
> BTW, I'm wondering whether and how non-root qemu works with xl disk
> configuration that is going to access a host block device, e.g.
>      disk = [ '/dev/sdb,,hda' ]
> If that works with non-root qemu, I may take the similar solution for
> pmem.
 
Today the user is required to give the correct ownership and access mode
to the block device, so that non-root QEMU can open it. However in the
case of PCI passthrough, QEMU needs to mmap /dev/mem, as a consequence
the feature doesn't work at all with non-root QEMU
(http://marc.info/?l=xen-devel&m=145261763600528).

If there is one /dev/pmem device per assignable region, then it would be
conceivable to change its ownership so that non-root QEMU can open it.
Or, better, the file descriptor could be passed by the toolstack via
qmp.

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