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

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@lists.xen.org] On Behalf Of
> Paul Durrant
> Sent: 22 March 2017 16:32
> To: Ian Jackson <Ian.Jackson@citrix.com>
> Cc: xen-devel@lists.xenproject.org; Wei Liu <wei.liu2@citrix.com>
> Subject: Re: [Xen-devel] [PATCH RESEND] tools/libxl: add support for
> emulated NVMe drives
> 
> > -----Original Message-----
> > From: Ian Jackson [mailto:ian.jackson@eu.citrix.com]
> > Sent: 22 March 2017 16:03
> > To: Paul Durrant <Paul.Durrant@citrix.com>
> > Cc: xen-devel@lists.xenproject.org; Wei Liu <wei.liu2@citrix.com>
> > 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"):
> > > > Oh, wait, I have just noticed that you have reused an entry in this
> > > > table!
> > > >
> > > >        1 << 28 | disk << 8 | partition   xvd, disks or partitions 16 onwards
> > > > ...
> > > > > +    1 << 28 | disk << 8               nvme, all disks, whole disk only
> > >
> > > Yes, that was intentional. No need for a new concrete encoding if we
> > > ignore namespaces, as I said in the commit comment. If you want to
> > > support namespaces then it would need something new... which doesn't
> > > seem worth it since QEMU has no support.
> >
> > 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.
> 
> >
> > We can't have one single numerical encoding mapping to two device
> > names.  That makes no sense.
> >
> > 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.
> 
> > In which case guests need to
> > be told that the device name for the vbd should be "nvme<something>",
> > which can only be done by using a different number.
> >
> 
> 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.
> 

Also, if we're going to go for distinct numbering then I guess the QEMU patch is probably wrong because we'd want guests with non-aware PV frontend to leave emulated NVMe devices plugged in.

  Paul

>   Paul
> 
> > Ian.
> 
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