Re: [PATCH v2 0/4] hw/nvme: fix controller hotplugging

[Klaus Jensen]

On Jul  9 08:16, Hannes Reinecke wrote:
> On 7/9/21 8:05 AM, Klaus Jensen wrote:
> > On Jul  7 17:49, Klaus Jensen wrote:
> > > From: Klaus Jensen <k.jensen@samsung.com>
> > >
> > > Back in May, Hannes posted a fix[1] to re-enable NVMe PCI hotplug. We
> > > discussed a bit back and fourth and I mentioned that the core issue was
> > > an artifact of the parent/child relationship stemming from the qdev
> > > setup we have with namespaces attaching to controller through a qdev
> > > bus.
> > >
> > > The gist of this series is the fourth patch "hw/nvme: fix controller hot
> > > unplugging" which basically causes namespaces to be reassigned to a bus
> > > owned by the subsystem if the parent controller is linked to one. This
> > > fixes `device_del/add nvme` in such settings.
> > >
> > > Note, that in the case that there is no subsystem involved, nvme devices
> > > can be removed from the system with `device_del`, but this *will* cause
> > > the namespaces to be removed as well since there is no place (i.e. no
> > > subsystem) for them to "linger". And since this series does not add
> > > support for hotplugging nvme-ns devices, while an nvme device can be
> > > readded, no namespaces can. Support for hotplugging nvme-ns devices is
> > > present in [1], but I'd rather not add that since I think '-device
> > > nvme-ns' is already a bad design choice.
> > >
> > > Now, I do realize that it is not "pretty" to explicitly change the
> > > parent bus, so I do have a an RFC patch in queue that replaces the
> > > subsystem and namespace devices with objects, but keeps -device shims
> > > available for backwards compatibility. This approach will solve the
> > > problems properly and should be a better model. However, I don't believe
> > > it will make it for 6.1 and I'd really like to at least fix the
> > > unplugging for 6.1 and this gets the job done.
> > >
> > >  [1]: 20210511073511.32511-1-hare@suse.de
> > >
> > > v2:
> > > - added R-b's by Hannes for patches 1 through 3
> > > - simplified "hw/nvme: fix controller hot unplugging"
> > >
> > > Klaus Jensen (4):
> > >  hw/nvme: remove NvmeCtrl parameter from ns setup/check functions
> > >  hw/nvme: mark nvme-subsys non-hotpluggable
> > >  hw/nvme: unregister controller with subsystem at exit
> > >  hw/nvme: fix controller hot unplugging
> > >
> > > hw/nvme/nvme.h   | 18 +++++++++-------
> > > hw/nvme/ctrl.c   | 14 ++++++------
> > > hw/nvme/ns.c     | 55 +++++++++++++++++++++++++++++++-----------------
> > > hw/nvme/subsys.c |  9 ++++++++
> > > 4 files changed, 63 insertions(+), 33 deletions(-)
> > >
> > > --
> > > 2.32.0
> >
> > Applied patches 1 through 3 to nvme-next.
>
> So, how do we go about with patch 4?
> Without it this whole exercise is a bit pointless, seeing that it 
> doesn't fix anything.

Patch 1-3 are fixes we need anyway, so I thought I might as well apply 
them :)

> Shall we go with that patch as an interim solution?
> Will you replace it with your 'object' patch?
> What is the plan?

Yes, if acceptable, I would like to use patch 4 as an interim solution. 
We have a bug we need to fix for 6.1, and I belive this does the job.

I considered changing the existing nvme-bus to be on the main system 
bus, but then we break the existing behavior that the namespaces attach 
to the most recently defined controller in the absence of the shared 
parameter or an explicit bus parameter.

Wrt. "the plan", right now, I see two solutions going forward:

1. Introduce new -object's for nvme-nvm-subsystem and nvme-ns
   This is the approach that I am taking right now and it works well. It 
   allows many-to-many relationships and separates the life times of 
   subsystems, namespaces and controllers like you mentioned.

   Conceptually, I also really like that the subsystem and namespace are 
   not "devices". One could argue that the namespace is comparable to a 
   SCSI LUN (-device scsi-hd, right?), but where the SCSI LUN actually 
   "shows up" in the host, the nvme namespace does not.

   My series handles backwards compatibility by keeping -device "shims" 
   around that just wraps the new objects but behaves like it used to. 
   The plan would be to deprecate these devices.

   The downside to this approach is that it moves the subsystem and 
   namespaces out of the "qdev tree (info qtree)" and into the pure QOM 
   "/objects" tree. Instead of qtree, we can have QMP and HMP commands 
   for introspection.

2. Make the subsystem a "system bus device"
   This way we add an "nvme-nvm-subsystems" bus as a direct child of the 
   main system bus, and we can possibly get rid of the explicit -device 
   nvme-subsys as well. We change the namespace device to plug into that 
   instead. The nvme controller device still needs to plug into the PCI 
   bus, so it cannot be a child of the subsystems bus, but can keep 
   using a link parameter to hook into the subsystem and attach to any 
   namespaces it would like.

   I'm unsure if we can do this without deprecating the existing 
   namespace device, just like option 1.

   I have not implemented this, so I need to look more into it. It seems 
   like the main thing that this gives us compared to 1) is `info qtree` 
   support and we still end up just "wiring" namespace attachment with 
   backlinks anyway.

I'm not sure what I would prefer, but I've found that implemting it as 
-object's is a breath of fresh air and as I mentioned, conceptually, I 
like option 1 because namespaces are -objects and not -devices.

And, by the way, thanks for chipping in on this Hannes, I had sort of 
crossed off option 2 before you showed up and threw some ideas in the 
air ;)

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