mshv: Enable SMT for guests with threads_per_core > 1 by weltling · Pull Request #7668 · cloud-hypervisor/cloud-hypervisor · GitHub
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mshv: Enable SMT for guests with threads_per_core > 1#7668

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weltling:mshv-smt-enabled-guest
Feb 7, 2026
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mshv: Enable SMT for guests with threads_per_core > 1#7668
rbradford merged 2 commits intocloud-hypervisor:mainfrom
weltling:mshv-smt-enabled-guest

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@weltling weltling commented Feb 5, 2026 •
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Set HV_PARTITION_CREATION_FLAG_SMT_ENABLED_GUEST when the guest topology
has more than one thread per core. This allows the hypervisor to schedule
guest VPs correctly on SMT-enabled hosts.

Without this flag, the hypervisor schedules guest VPs incorrectly,
causing SMT unusable.

Includes also the updated MSHV crates as a dependency.

This complements #7576 by ensuring SMT is enabled when the topology configured accordingly.

@weltling weltling requested a review from a team as a code owner February 5, 2026 11:10
@weltling weltling force-pushed the mshv-smt-enabled-guest branch from 5617fe2 to 03ba4e8 Compare February 5, 2026 11:44
phip1611
phip1611 requested changes Feb 5, 2026
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LGTM! One remark tho

Comment thread hypervisor/src/mshv/mod.rs
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since it is no longer unused, please rename _config to config

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@weltling weltling Feb 5, 2026

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Indeed. There are some other struct members hanging on it, too. I made a dedicated commit, it doesn't seem to deserve a dedicated PR.

Thanks

rbradford
rbradford reviewed Feb 5, 2026
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Comment thread Cargo.toml
linux-loader = "0.13.1"
mshv-bindings = "0.6.6"
mshv-ioctls = "0.6.6"
mshv-bindings = "0.6.7"
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@rbradford rbradford Feb 5, 2026

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Nice to see 👍 but you need to check in the Cargo.lock with it.

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@weltling weltling Feb 5, 2026

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Fixed, thanks for the hint.

@weltling
build: Update mshv crates to 0.6.7
c0d0b41 
Release notes: rust-vmm/mshv#307

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
@weltling weltling force-pushed the mshv-smt-enabled-guest branch from 03ba4e8 to 712665c Compare February 5, 2026 13:48
@weltling weltling force-pushed the mshv-smt-enabled-guest branch from 712665c to 0fd3e09 Compare February 5, 2026 13:53
@rbradford rbradford enabled auto-merge February 5, 2026 13:54
auto-merge was automatically disabled February 5, 2026 13:55

Head branch was pushed to by a user without write access

@weltling weltling force-pushed the mshv-smt-enabled-guest branch from 0fd3e09 to e058a96 Compare February 5, 2026 13:55
phip1611
phip1611 reviewed Feb 5, 2026
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Comment thread hypervisor/src/mshv/mod.rs Outdated
/// let vm = hypervisor.create_vm(config).unwrap();
/// ```
fn create_vm(&self, _config: HypervisorVmConfig) -> hypervisor::Result<Arc<dyn vm::Vm>> {
fn create_vm(&self, config: HypervisorVmConfig) -> hypervisor::Result<Arc<dyn vm::Vm>> {
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@phip1611 phip1611 Feb 5, 2026 •
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ah shoot, clippy is failing as with some cargo feature configurations, this variable is unused 🙄

Hm..... yes, than maybe sticking to _config might be the pragmatic solution, although it isn't very nice

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@rbradford rbradford Feb 5, 2026 •
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@weltling Do you want to just drop the the last commit then?

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@weltling weltling Feb 5, 2026

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I went with this, but otherwise seems no ideal solution to this.

#[cfg(any(feature = "sev_snp", target_arch = "x86_64"))] config: HypervisorVmConfig,
#[cfg(not(any(feature = "sev_snp", target_arch = "x86_64")))] _config: HypervisorVmConfig,

If this doesn't work out, I'll drop the last commit then.

Thanks

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@weltling weltling Feb 5, 2026

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Looks like clippy is happy with that :)

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Thanks for investigating! I think keeping _config is the lesser evil here

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@weltling weltling Feb 5, 2026

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Gotcha, I dropped that. The compiler seems also to handle it correctly disreganding _. Thanks

@weltling weltling force-pushed the mshv-smt-enabled-guest branch from e058a96 to a11eed7 Compare February 5, 2026 18:49
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weltling commented Feb 5, 2026

Hmm, looks like the MSHV CI runner might not have an updated host kernel version. Looking at what it says:

VmCreate(Kernel returned errno: Invalid argument (os error 22)) at vmm/src/vm.rs:1150

this likely means tests test_cpu_topology_262 and test_cpu_topology_421 are failing due to smt_enabled = true. @anirudhrb could you please confirm CI is using the latest kernel version?

Thanks!

@weltling weltling force-pushed the mshv-smt-enabled-guest branch from a11eed7 to 34565ab Compare February 5, 2026 22:26
russell-islam
russell-islam approved these changes Feb 6, 2026
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LGTM

@weltling weltling force-pushed the mshv-smt-enabled-guest branch from 34565ab to 134480a Compare February 6, 2026 11:30
@weltling
hypervisor: vmm: mshv: Enable SMT for guests with threads_per_core > 1
1d5d8fd 
Set HV_PARTITION_CREATION_FLAG_SMT_ENABLED_GUEST when the guest
topology has more than one thread per core. This allows the
hypervisor to schedule guest VPs correctly on SMT-enabled hosts.

Without this flag, the hypervisor schedules guest VPs incorrectly,
causing SMT unusable.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
@weltling weltling force-pushed the mshv-smt-enabled-guest branch from 134480a to 1d5d8fd Compare February 6, 2026 17:26
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weltling commented Feb 6, 2026

@rbradford rbradford added this pull request to the merge queue Feb 7, 2026
Merged via the queue into cloud-hypervisor:main with commit a00189c Feb 7, 2026
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@weltling weltling deleted the mshv-smt-enabled-guest branch February 8, 2026 18:48
@likebreath likebreath added the mshv Affects mshv only label Feb 19, 2026
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