Re: [Xen-devel] [PATCH RESEND] tools/libxl: add support for emulated NVMe drives

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Ian Jackson [mailto:ian.jackson@xxxxxxxxxxxxx]
> Sent: 22 March 2017 17:03
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Wei Liu <wei.liu2@xxxxxxxxxx>
> Subject: RE: [PATCH RESEND] tools/libxl: add support for emulated NVMe
> drives
> 
> Paul Durrant writes ("RE: [PATCH RESEND] tools/libxl: add support for
> emulated NVMe drives"):
> > > From: Ian Jackson [mailto:ian.jackson@xxxxxxxxxxxxx]
> > > That's not my point.  The purpose of this table is to advise guests
> > > what the conventional in-guest device name ought to be for a certain
> > > vbd.
> >
> > Yes, and xvd<something> is a perfectly fine name for a PV device in pretty
> much every case. It's already the case that emulated IDE disks are exposed to
> guests using xvd* numbering.
> 
> No, I don't think so:
> 
> /libxl/5/device/vbd/5632/params = "aio:/root/68254.test-amd64-amd64-xl-
> qemuu-debianhvm-amd64.debianhvm-em\..."
> (n0)
> 
> 5632 = 22 << 8 | 0 ie "hd, disk 2, partition 0"
> 

This is my VM:

root@brixham:~# xenstore-ls "/libxl/3"
device = ""
 vbd = ""
  51712 = ""
   frontend = "/local/domain/3/device/vbd/51712"
   backend = "/local/domain/0/backend/qdisk/3/51712"
   params = "qcow2:/root/winrs2-pv1.qcow2"
   frontend-id = "3"
   online = "1"
   removable = "0"
   bootable = "1"
   state = "1"
   dev = "xvda"
   type = "qdisk"
   mode = "w"
   device-type = "disk"
   discard-enable = "1"

No problem using xvda... still ends up as IDE primary master.

> Some operating systems (including many recent Linux kernels) present
> all vbds as xvd*.
> 
> > > Presumably these NVME devices should be subject to the same vbd and
> > > unplug approach as scsi and ide disks.
> >
> > Yes, that's what the QEMU patch does.
> 
> So maybe they should reuse the hd* numbering ?
> 

That might be too limiting. The hd* numbering scheme doesn't stretch very far.

> > That means modifications to PV frontends would be needed, which is
> > going to make things more difficult. Most OS find disks by UUID
> > these days anyway so I'm still not sure that just using xvd*
> > numbering would really be a problem.
> 
> In terms of the "nominal disk type" discussed in
> xen-vbd-interface.markdown.7, I don't think these emulated devices,
> which get unplugged, should be have a "nomainl disk type" of "Xen
> virtual disk".
> 

Ok. I'll submit another patch to QEMU to distinguish between IDE/SCSI disks and 
NVMe disks in the unplug protocol, come up with a new PV numbering schemed and 
modify the Windows frontend to understand it.

  Paul

> Ian.

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