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


From: Klaus Jensen
Subject: Re: [PATCH RFC 00/13] hw/nvme: experimental user-creatable objects
Date: Fri, 17 Sep 2021 08:21:59 +0200

On Sep 16 18:30, Klaus Jensen wrote:
> 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.
> 

There is also the issue that the x-nvme-ns-nvm -object needs a blockdev
- and the ordering is also a problem here. That also requires the
machine done notifier.

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