aarch64: expose SMT topology in cpu-map and allow smt: true - #6154
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NathanChenNVIDIA wants to merge 5 commits into
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aarch64: expose SMT topology in cpu-map and allow smt: true#6154NathanChenNVIDIA wants to merge 5 commits into
NathanChenNVIDIA wants to merge 5 commits into
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Emit a minimal cpu-map -- one cluster containing N cores -- and give each cpu node an explicit phandle so the cluster/core entries can reference it. Signed-off-by: Walter Wang <walterw@nvidia.com>
Extend the cpu-map node so that, when SMT is enabled, consecutive vCPUs (2*i, 2*i+1) are emitted as the two `thread` nodes of core `i` instead of as two independent cores. arm64 Linux derives thread siblings and "Thread(s) per core" purely from the device tree: parse_core() in drivers/base/arch_topology.c reads the `thread%d` child nodes, and store_cpu_topology() -- the fallback used when no cpu-map is present -- hardcodes thread_id to -1. MPIDR_EL1 is not consulted, so the cpu-map node alone is sufficient to expose SMT and no vCPU register state needs to change. With SMT disabled, or with a single vCPU, the emitted tree is unchanged. Signed-off-by: Nathan Chen <nathanc@nvidia.com>
Remove the aarch64-specific rejection of smt: true in machine-config updates. The same validation rules as x86_64 now apply: vcpu_count must be 1 or even when SMT is enabled. Update the affected tests, API documentation, and changelog accordingly. Signed-off-by: Nathan Chen <nathanc@nvidia.com>
Update integration tests to verify SMT can be configured on aarch64 and that guest CPU topology reflects the configured thread count. Add focused 2-vCPU and 4-vCPU cases that assert two threads per core. Signed-off-by: Nathan Chen <nathanc@nvidia.com>
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Changes
Reason
aarch64 guests currently have no cpu-map, so Linux treats each vCPU as its own core (thread_id = -1). SMT pairing on arm64 comes from the device tree (parse_core() in arch_topology.c), not from MPIDR, so describing thread siblings in cpu-map is enough for the guest to show a real SMT topology. The API already had an smt flag; rejecting it only on aarch64 blocked that topology from being configured.
Testing
On aarch64, a 2-vCPU guest with
smt: trueand guest kernel 6.1 reports:lscpu: CPU(s)=2, Thread(s) per core=2, Core(s) per cluster=1, Cluster(s)=1/sys/devices/system/cpu/cpu0/topology/thread_siblings_list:0-1/proc/device-tree/cpus/cpu-map/cluster0/core0/:thread0,thread1test_cpu_topologyskips on this host (kernel ≥ 6.14). The new ARM hwlocexpectations in that helper have not run here.
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how they are solving the problem in a clear and encompassing way.
in the PR.
CHANGELOG.md.If a specific issue led to this PR, this PR closes the issue.Runbook for Firecracker API changes.
integration tests.
I have linked an issue to every newTODO.rust-vmm.