On Wed, 20 Oct 2021 16:15:29 +0200 Michal Prívozník <mprivozn@redhat.com> wrote:
On 10/20/21 1:18 PM, Peter Krempa wrote:
On Wed, Oct 20, 2021 at 13:07:59 +0200, Michal Prívozník wrote:
On 10/6/21 3:32 PM, Igor Mammedov wrote:
On Thu, 30 Sep 2021 14:08:34 +0200 Peter Krempa <pkrempa@redhat.com> wrote:
[...]
2) In my experiments I try to mimic what libvirt does. Here's my cmd line:
qemu-system-x86_64 \ -S \ -preconfig \ -cpu host \ -smp 120,sockets=2,dies=3,cores=4,threads=5 \ -object '{"qom-type":"memory-backend-memfd","id":"ram-node0","size":4294967296,"host-nodes":[0],"policy":"bind"}' \ -numa node,nodeid=0,memdev=ram-node0 \ -no-user-config \ -nodefaults \ -no-shutdown \ -qmp stdio
and here is my QMP log:
{"QMP": {"version": {"qemu": {"micro": 50, "minor": 1, "major": 6}, "package": "v6.1.0-1552-g362534a643"}, "capabilities": ["oob"]}}
{"execute":"qmp_capabilities"} {"return": {}}
{"execute":"query-hotpluggable-cpus"} {"return": [{"props": {"core-id": 3, "thread-id": 4, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, {"props": {"core-id": 3, "thread-id": 3, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, {"props": {"core-id": 3, "thread-id": 2, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, {"props": {"core-id": 3, "thread-id": 1, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, {"props": {"core-id": 3, "thread-id": 0, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, {"props": {"core-id": 2, "thread-id": 4, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, <snip/> {"props": {"core-id": 0, "thread-id": 0, "die-id": 0, "socket-id": 0}, "vcpus-count": 1, "type": "host-x86_64-cpu"}]}
I can see that query-hotpluggable-cpus returns an array. Can I safely assume that vCPU ID == index in the array? I mean, if I did have -numa
No, this assumption would be incorrect on the aforementioned PPC platform where one entry in the returned array can describe multiple cores.
qemuDomainFilterHotplugVcpuEntities is the code that cross-references the libvirt "index" with the data returned by query-hotpluggable cpus.
The important bit is the 'vcpus-count' property. The code which deals with hotplug is already fetching everything that's needed.
Ah, I see. So my assumption would be correct if vcpus-count would be 1 for all entries. If it isn't then I need to account for how much only for some boards. An entry in array describes an single entity that should be handled as a single device by user (-device/plug/unplug/other mapping options) (and the entity might have 1 or more vCPUs (threads) depending on target arch/board).
vcpus-count is in each entity. Fair enough. But qemuDomainFilterHotplugVcpuEntities() doesn't really do vCPU ID -> [socket, core, thread] translation, does it?
But even if it did, I am still wondering what the purpose of this whole exercise is. QEMU won't be able to drop ID -> [socket, core, thread] mapping. The only thing it would be able to drop is a few lines of code handling command line. Am I missing something obvious? I described in other email why QEMU is dropping cpu_idex on external interfaces (it's possible to drop it internally too, but I don't see much gain there vs effort such refactoring would require).
Sure thing, you can invent/maintain libvirt internal "vCPU ID" -> [topo props] mapping if it's necessary. However using just a "vCPU ID" will obscure topology information from upper layers. Maybe providing a list of CPUs as an external interface would be better, then user can pick up which CPUs they wish to add/delete/assign/... using items from that list.
Michal