I'm about to drop off for two weeks of much-needed vacation. I meant to study your explanation and give design advice before I leave, but I'm out of time. Regrettable. I hope Stefan can help you. Or perhaps Paolo. If you still have questions when I'm back, feel free to contact me again.
Klaus Jensen <i...@irrelevant.dk> writes: > On May 12 14:02, Markus Armbruster wrote: >>Klaus Jensen <i...@irrelevant.dk> writes: >> >>> Hi all, >>> >>> I need some help with grok'ing qdev busses. Stefan, Michael - David >>> suggested on IRC that I CC'ed you guys since you might have solved a >>> similar issue with virtio devices. I've tried to study how that works, >>> but I'm not exactly sure how to apply it to the issue I'm having. >>> >>> Currently, to support multiple namespaces on the emulated nvme device, >>> one can do something like this: >>> >>> -device nvme,id=nvme-ctrl-0,serial=foo,... >>> -device nvme-ns,id=nvme-ns-0,bus=nvme-ctrl-0,... >>> -device nvme-ns,id-nvme-ns-1,bus=nvme-ctrl-0,... >>> >>> The nvme device creates an 'nvme-bus' and the nvme-ns devices has >>> dc->bus_type = TYPE_NVME_BUS. This all works very well and provides a >>> nice overview in `info qtree`: >>> >>> bus: main-system-bus >>> type System >>> ... >>> dev: q35-pcihost, id "" >>> .. >>> bus: pcie.0 >>> type PCIE >>> .. >>> dev: nvme, id "nvme-ctrl-0" >>> .. >>> bus: nvme-ctrl-0 >>> type nvme-bus >>> dev: nvme-ns, id "nvme-ns-0" >>> .. >>> dev: nvme-ns, id "nvme-ns-1" >>> .. >>> >>> >>> Nice and qdevy. >>> >>> We have since introduced support for NVM Subsystems through an >>> nvme-subsys device. The nvme-subsys device is just a TYPE_DEVICE and >>> does not show in `info qtree` >> >>Yes. >> >>Most devices plug into a bus. DeviceClass member @bus_type specifies >>the type of bus they plug into, and DeviceState member @parent_bus >>points to the actual BusState. Example: PCI devices plug into a PCI >>bus, and have ->bus_type = TYPE_PCI_BUS. >> >>Some devices don't. @bus_type and @parent_bus are NULL then. >> >>Most buses are provided by a device. BusState member @parent points to >>the device. >> >>The main-system-bus isn't. Its @parent is null. >> >>"info qtree" only shows the qtree rooted at main-system-bus. It doesn't >>show qtrees rooted at bus-less devices or device-less buses other than >>main-system-bus. I doubt such buses exist. >> > > Makes sense. > >>> (I wonder if this should actually just >>> have been an -object?). >> >>Does nvme-subsys expose virtual hardware to the guest? Memory, IRQs, >>... >> >>If yes, it needs to be a device. >> >>If no, object may be more appropriate. Tell us more about what it does. >> > > It does not expose any virtual hardware. See below. > >> >>> Anyway. The nvme device has a 'subsys' link >>> parameter and we use this to manage the namespaces across the >>> subsystem that may contain several nvme devices (controllers). The >>> problem is that this doesnt work too well with unplugging since if the >>> nvme device is `device_del`'ed, the nvme-ns devices on the nvme-bus >>> are unrealized which is not what we want. We really want the >>> namespaces to linger, preferably on an nvme-bus of the nvme-subsys >>> device so they can be attached to other nvme devices that may show up >>> (or already exist) in the subsystem. >>> >>> The core problem I'm having is that I can't seem to create an nvme-bus >>> from the nvme-subsys device and make it available to the nvme-ns >>> device on the command line: >>> >>> -device nvme-subsys,id=nvme-subsys-0,... >>> -device nvme-ns,bus=nvme-subsys-0 >>> >>> The above results in 'No 'nvme-bus' bus found for device 'nvme-ns', >>> even though I do `qbus_create_inplace()` just like the nvme >>> device. However, I *can* reparent the nvme-ns device in its realize() >>> method, so if I instead define it like so: >>> >>> -device nvme-subsys,id=nvme-subsys-0,... >>> -device nvme,id=nvme-ctrl-0,subsys=nvme-subsys-0 >>> -device nvme-ns,bus=nvme-ctrl-0 >>> >>> I can then call `qdev_set_parent_bus()` and set the parent bus to the >>> bus creates in the nvme-subsys device. This solves the problem since >>> the namespaces are not "garbage collected" when the nvme device is >>> removed, but it just feels wrong you know? Also, if possible, I'd of >>> course really like to retain the nice entries in `info qtree`. >> >>I'm afraid I'm too ignorant on NVME to give useful advice. >> >>Can you give us a brief primer on the aspects of physical NVME devices >>you'd like to model in QEMU? What are "controllers", "namespaces", and >>"subsystems", and how do they work together? >> >>Once we understand the relevant aspects of physical devices, we can >>discuss how to best model them in QEMU. >> > > An "NVM Subsystem" is basically just a term to talk about a collection > of controllers and namespaces. A namespace is just a quantity of > non-volatile memory that the controller can use to store stuff on. > > Only the controller is a piece of virtual hardware. An example > subsystem looks like this: > > > +------------------+ +-----------------+ > | controller A | | controller B | > +------------------+ +-----------------+ > +--------++--------+ +--------++-------+ > | NSID 1 || NSID 2 | | NSID 3 | NSID 2 | > +--------++--------+ +--------++-------+ > +--------+ | +--------+ | > | NS A | | | NS C | | > +--------+ | +--------+ | > | | > +------------------------+ > | > +--------+ > | NS B | > +--------+ > > > This is the example in Figure 5 in the NVMe v1.4 specification. Here, > we have two controllers (that we model with the 'nvme' pci-based > device). Each controller has one "private" namespace (NS A and NS C) > and shares one namespace (NS B). The namespace IDs are unique across > the subsystem and are assigned by the controller when attached to a > namespace. > > We use the 'nvme-ns' device (TYPE_DEVICE) to model the namespaces, and > I guess this should could also just have been an -object, not sure if > we can change that now. The 'nvme-ns' device mostly exist to hold the > block backend configuration and related namespace only > parameters. Prior to the introduction of subsystem, while we could > have multiple controllers on the PCI bus, they could not share > namespaces. To support this we introduced the 'nvme-subsys' device to > allow the namespaces to be shared. This support is considered > experimental, so I think we can get away with changing this to be an > object. > > As I explained in my first mail, we attach namespaces to controllers > through a bus. This means that even in the absence of an explicit > "bus=..." parameter on the nvme-ns device, it will "connect" on the > most recently defined "nvme-bus" (of the most recently defined > controller). With subsystems we would also like to model "unattached" > namespaces that exists solely in the subsystem (i.e. NOT attached to > any controllers). That is why I was trying to get the nvme-ns devices > to attach to a bus created by the "non-bus-attached" subsystem > device. And that is what I can't do. We could add a link property to > the nvme-ns device instead, but then the bus magic in qemu would still > happen and the namespace would end up "attached" (in qemu terms) to a > controller anyway - and it would complain if we defined the namespace > device prior to defining any controller devices since no usable bus > exist. > > Thanks for helping out with this!
