Re: [PATCH RFC 00/13] hw/nvme: experimental user-creatable objects

[Klaus Jensen <its@irrelevant.dk>]

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On Sep 16 14:41, Kevin Wolf wrote:
> Am 14.09.2021 um 22:37 hat Klaus Jensen geschrieben:
> > From: Klaus Jensen <k.jensen@samsung.com>
> > 
> > Hi,
> > 
> > This is an attempt at adressing a bunch of issues that have presented
> > themselves since we added subsystem support. It's been brewing for a
> > while now.
> > 
> > Fundamentally, I've come to the conclusion that modeling namespaces and
> > subsystems as "devices" is wrong. They should have been user-creatable
> > objects. We've run into multiple issues with wrt. hotplugging due to how
> > namespaces hook up to the controller with a bus. The bus-based design
> > made a lot of sense when we didn't have subsystem support and it follows
> > the design of hw/scsi. But, the problem here is that the bus-based
> > design dictates a one parent relationship, and with shared namespaces,
> > that is just not true. If the namespaces are considered to have a single
> > parent, that parent is the subsystem, not any specific controller.
> > 
> > This series adds a set of experimental user-creatable objects:
> > 
> >   -object x-nvme-subsystem
> >   -object x-nvme-ns-nvm
> >   -object x-nvme-ns-zoned
> > 
> > It also adds a new controller device (-device x-nvme-ctrl) that supports
> > these new objects (and gets rid of a bunch of deprecated and confusing
> > parameters). This new approach has a bunch of benefits (other than just
> > fixing the hotplugging issues properly) - we also get support for some
> > nice introspection through some new dynamic properties:
> > 
> >   (qemu) qom-get /machine/peripheral/nvme-ctrl-1 attached-namespaces
> >   [
> >       "/objects/nvm-1",
> >       "/objects/zns-1"
> >   ]
> > 
> >   (qemu) qom-list /objects/zns-1
> >   type (string)
> >   subsys (link<x-nvme-subsystem>)
> >   nsid (uint32)
> >   uuid (string)
> >   attached-ctrls (str)
> >   eui64 (string)
> >   blockdev (string)
> >   pi-first (bool)
> >   pi-type (NvmeProtInfoType)
> >   extended-lba (bool)
> >   metadata-size (uint16)
> >   lba-size (size)
> >   zone-descriptor-extension-size (size)
> >   zone-cross-read (bool)
> >   zone-max-open (uint32)
> >   zone-capacity (size)
> >   zone-size (size)
> >   zone-max-active (uint32)
> > 
> >   (qemu) qom-get /objects/zns-1 pi-type
> >   "none"
> > 
> >   (qemu) qom-get /objects/zns-1 eui64
> >   "52:54:00:17:67:a0:40:15"
> > 
> >   (qemu) qom-get /objects/zns-1 zone-capacity
> >   12582912
> > 
> > Currently, there are no shortcuts, so you have to define the full
> > topology to get it up and running. Notice that the topology is explicit
> > (the 'subsys' and 'attached-ctrls' links). There are no 'nvme-bus'
> > anymore.
> > 
> >   -object x-nvme-subsystem,id=subsys0,subnqn=foo
> >   -device x-nvme-ctrl,id=nvme-ctrl-0,serial=foo,subsys=subsys0
> >   -device x-nvme-ctrl,id=nvme-ctrl-1,serial=bar,subsys=subsys0
> >   -drive  id=nvm-1,file=nvm-1.img,format=raw,if=none,discard=unmap
> >   -object x-nvme-ns-nvm,id=nvm-1,blockdev=nvm-1,nsid=1,subsys=subsys0,attached-ctrls=nvme-ctrl-1
> >   -drive  id=nvm-2,file=nvm-2.img,format=raw,if=none,discard=unmap
> >   -object x-nvme-ns-nvm,id=nvm-2,blockdev=nvm-2,nsid=2,subsys=subsys0,attached-ctrls=nvme-ctrl-0
> 
> I may be wrong here, but my first gut feeling when seeing this was that
> referencing the controller device in the namespace object feels
> backwards. Usually, we have objects that are created independently and
> then the devices reference them.
> 
> Your need to use a machine_done notifier is probably related to that,
> too, because it goes against the normal initialisation order, so you
> have to wait. Error handling also isn't really possible in the notifier
> any more, so this series seems to just print something to stderr, but
> ignore the error otherwise.
> 
> Did you consider passing a list of namespaces to the controller device
> instead?
> 
> I guess a problem that you have with both ways is that support for
> list options isn't great in QemuOpts, which is still used both for
> -object and -device in the system emulator...
> 

Heh. Exactly. The ability to better support lists with -object through
QAPI is why I did it like this...

Having the list of namespaces on the controller is preferable. I'll see
what I can come up with.

Thanks!

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