Hi Daniel and folks, Let me pick up this thread again to discuss some updates from my work on QOM CPU topology. I would like to know if I'm on the right track. :-)
On Mon, Feb 13, 2023 at 01:38:28PM +0000, Daniel P. Berrangé wrote: > [snip] > > IIUC the functionality offered by -hybrid should be a superset > of the -smp functionality. IOW, -smp ought to be possible to > re-implement -smp as an alias for -hybrid, such that internally > code only ever has to deal with the modern approach. Having to > keep support for both -smp and -hybrid throughout the code is > undesirable IMHO. Keeping the compat at the CLI parsing level > limits the burden. > > > As a more general thought, rather than introducing a new top level > command line argument -hybrid, I'm thinking we should possibly just > define this all using QOM and thus re-use the existing -object > argument. > > I'm also finding the above example command lines quite difficult > to understand, as there is alot of implicit linkage and expansion > between the different levels. With devices we're much more > explicitly with the parent/child relationships, and have to > express everything with no automatic expansion, linking it all > together via the id=/bus= properties. This is quite a bit more > verbose, but it is also very effective at letting us express > arbitrarily complex relationships. > > I think it would be worth exploring that approach for the CPU > topology expression too. > > If we followed the more explicit device approach to modelling > then instead of: > > -cpu core,... > -cpu atom,... > -hybrid socket,sockets=1 > -hybrid die,dies=1 > -hybrid cluster,clusters=4 > -hybrid core,cores=1,coretype="core",threads=2,clusterid=0-2 > -hybrid core,cores=4,coretype="atom",threads=1 > > we would end up with something like > > -object cpu-socket,id=sock0 > -object cpu-die,id=die0,parent=sock0 > -object cpu-cluster,id=cluster0,parent=die0 > -object cpu-cluster,id=cluster1,parent=die0 > -object cpu-cluster,id=cluster2,parent=die0 > -object cpu-cluster,id=cluster3,parent=die0 > -object x86-cpu-model-atom,id=cpu0,parent=cluster0 > -object x86-cpu-model-atom,id=cpu1,parent=cluster0 > -object x86-cpu-model-atom,id=cpu2,parent=cluster0 > -object x86-cpu-model-atom,id=cpu3,parent=cluster0 > -object x86-cpu-model-core,id=cpu4,parent=cluster0,threads=2 > -object x86-cpu-model-atom,id=cpu5,parent=cluster1 > -object x86-cpu-model-atom,id=cpu6,parent=cluster1 > -object x86-cpu-model-atom,id=cpu7,parent=cluster1 > -object x86-cpu-model-atom,id=cpu8,parent=cluster1 > -object x86-cpu-model-core,id=cpu9,parent=cluster1,threads=2 > -object x86-cpu-model-atom,id=cpu10,parent=cluster2 > -object x86-cpu-model-atom,id=cpu11,parent=cluster2 > -object x86-cpu-model-atom,id=cpu12,parent=cluster2 > -object x86-cpu-model-atom,id=cpu13,parent=cluster2 > -object x86-cpu-model-core,id=cpu14,parent=cluster2,threads=2 > -object x86-cpu-model-atom,id=cpu15,parent=cluster3 > -object x86-cpu-model-atom,id=cpu16,parent=cluster3 > -object x86-cpu-model-atom,id=cpu17,parent=cluster3 > -object x86-cpu-model-atom,id=cpu18,parent=cluster3 > -object x86-cpu-model-core,id=cpu19,parent=cluster3,threads=2 > > The really obvious downside is that it is much more verbose. I find the "core" and "cluster" already have the abstraction as the device, and Andreas also tried to abstract "socket" as the device. So I find in this QOM way, all CPU topology levels should be abstracted to devices and created via "-device". And I also need to extend "cluster" device to support VM case as a general CPU topology abstraction, not only just used in TCG emulation. I've done a preliminary POC so far, where I can create the CPU topology via "-device", and also added the ability to create the "child" relationship in "-device" (since neither the "link" relationship nor the bus looks like a good representation of the CPU hierarchy). There's an example: -device cpu-socket,id=sock0 \ -device cpu-die,id=die0,parent=sock0 \ -device cpu-die,id=die1,parent=sock0 \ -device cpu-cluster,id=cluster0,parent=die0 \ -device cpu-cluster,id=cluster1,parent=die0 \ -device cpu-cluster,id=cluster2,parent=die1 \ -device x86-intel-core,id=core0,parent=cluster0,threads=3 \ -device x86-intel-atom,id=core1,parent=cluster0,threads=2 \ -device x86-core,id=core2,parent=cluster1,threads=1 \ -device x86-intel-core,id=core3,parent=cluster2,threads=5 \ with the above format, I could build the the more accurate canonical path for CPUs like this: (QEMU) query-hotpluggable-cpus { "arguments": {}, "execute": "query-hotpluggable-cpus" } { "return": [ { "props": { "cluster-id": 0, "core-id": 0, "die-id": 1, "socket-id": 0, "thread-id": 4 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die1/cluster2/core3/host-x86_64-cpu[10]", "type": "host-x86_64-cpu", "vcpus-count": 1 }, { "props": { "cluster-id": 0, "core-id": 0, "die-id": 1, "socket-id": 0, "thread-id": 3 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die1/cluster2/core3/host-x86_64-cpu[9]", "type": "host-x86_64-cpu", "vcpus-count": 1 }, { "props": { "cluster-id": 0, "core-id": 0, "die-id": 1, "socket-id": 0, "thread-id": 2 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die1/cluster2/core3/host-x86_64-cpu[8]", "type": "host-x86_64-cpu", "vcpus-count": 1 }, { "props": { "cluster-id": 0, "core-id": 0, "die-id": 1, "socket-id": 0, "thread-id": 1 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die1/cluster2/core3/host-x86_64-cpu[7]", "type": "host-x86_64-cpu", "vcpus-count": 1 }, { "props": { "cluster-id": 0, "core-id": 0, "die-id": 1, "socket-id": 0, "thread-id": 0 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die1/cluster2/core3/host-x86_64-cpu[6]", "type": "host-x86_64-cpu", "vcpus-count": 1 }, { "props": { "cluster-id": 1, "core-id": 0, "die-id": 0, "socket-id": 0, "thread-id": 0 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die0/cluster1/core2/host-x86_64-cpu[5]", "type": "host-x86_64-cpu", "vcpus-count": 1 }, { "props": { "cluster-id": 0, "core-id": 1, "die-id": 0, "socket-id": 0, "thread-id": 1 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die0/cluster0/core1/host-x86_64-cpu[4]", "type": "host-x86_64-cpu", "vcpus-count": 1 }, { "props": { "cluster-id": 0, "core-id": 1, "die-id": 0, "socket-id": 0, "thread-id": 0 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die0/cluster0/core1/host-x86_64-cpu[3]", "type": "host-x86_64-cpu", "vcpus-count": 1 }, { "props": { "cluster-id": 0, "core-id": 0, "die-id": 0, "socket-id": 0, "thread-id": 2 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die0/cluster0/core0/host-x86_64-cpu[2]", "type": "host-x86_64-cpu", "vcpus-count": 1 }, { "props": { "cluster-id": 0, "core-id": 0, "die-id": 0, "socket-id": 0, "thread-id": 1 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die0/cluster0/core0/host-x86_64-cpu[1]", "type": "host-x86_64-cpu", "vcpus-count": 1 }, { "props": { "cluster-id": 0, "core-id": 0, "die-id": 0, "socket-id": 0, "thread-id": 0 }, "qom-path": "/machine/peripheral/cpu-slot/sock0/die0/cluster0/core0/host-x86_64-cpu[0]", "type": "host-x86_64-cpu", "vcpus-count": 1 } ] } Of course, I'm still a bit far from the full QOM topology support, but would like to hear comments at the POC stage as well! :-) Thanks and BR, Zhao > > This example only has 20 CPUs. For a VM with say 1000 CPUs > this will be very big, but that doesn't neccesarily make it > wrong. > > On the flipside > > * It is really clear exactly how many CPUs I've added > > * The relationship between the topology levels is clear > > * Every CPU has a unique ID given that can be used in > later QMP commands > > * Whether or not 'threads' are permitted is now a property > of the specific CPU model implementation, not the global > config. IOW we can express that some CPU models allowing > for threads, and some don't. > > * The -cpu arg is also obsoleted, replaced by the > -object x86-cpu-model-core. This might facilitate the > modelling of machines with CPUs from different architectures. > > > We could potentially compress the leaf node level by expressing > how many instances of an object we want. it we want. ie, define > a more convenient shorthand syntax to creating many instances of > an object. so eg > > -object-set $TYPE,$PROPS,idbase=foo,count=4 > > would be functionally identical to > > -object $TYPE,$PROPS,id=foo.0 > -object $TYPE,$PROPS,id=foo.1 > -object $TYPE,$PROPS,id=foo.2 > -object $TYPE,$PROPS,id=foo.3 > > QEMU just expands it and creates all the objects internally. > > So the huge example I have above for 20 cpus would become much > shorter: e.g. > > -object cpu-socket,id=sock0 > -object cpu-die,id=die0,parent=sock0 > -object cpu-cluster,id=cluster0,parent=die0 > -object cpu-cluster,id=cluster1,parent=die0 > -object cpu-cluster,id=cluster2,parent=die0 > -object cpu-cluster,id=cluster3,parent=die0 > -object-set x86-cpu-core-atom,idbase=cpu0,parent=cluster0,count=4 > -object-set x86-cpu-core-core,id=cpu1,parent=cluster0,threads=2,count=1 > -object-set x86-cpu-core-atom,idbase=cpu2,parent=cluster1,count=4 > -object-set x86-cpu-core-core,id=cpu3,parent=cluster1,threads=2,count=1 > -object-set x86-cpu-core-atom,idbase=cpu4,parent=cluster2,count=4 > -object-set x86-cpu-core-core,id=cpu5,parent=cluster2,threads=2,count=1 > -object-set x86-cpu-core-atom,idbase=cpu6,parent=cluster3,count=4 > -object-set x86-cpu-core-core,id=cpu7,parent=cluster3,threads=2,count=1 > > IOW, the size of the CLI config only depends on the number of elements > in the hierarchy, and is independant of the number of leaf CPU cores. > > Obviously in describing all of the above, I've ignored any complexity > of dealing with our existing code implementation and pain of getting > it converted to the new model. > > With regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| >