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Hyper-V TLFS specifies this enlightenment as: "NoNonArchitecturalCoreSharing - Indicates that a virtual processor will never share a physical core with another virtual processor, except for virtual processors that are reported as sibling SMT threads. This can be used as an optimization to avoid the performance overhead of STIBP". However, STIBP is not the only implication. It was found that Hyper-V on KVM doesn't pass MD_CLEAR bit to its guests if it doesn't see NoNonArchitecturalCoreSharing bit. KVM reports NoNonArchitecturalCoreSharing in KVM_GET_SUPPORTED_HV_CPUID to indicate that SMT on the host is impossible (not supported of forcefully disabled). Implement NoNonArchitecturalCoreSharing support in QEMU as tristate: 'off' - the feature is disabled (default) 'on' - the feature is enabled. This is only safe if vCPUS are properly pinned and correct topology is exposed. As CPU pinning is done outside of QEMU the enablement decision will be made on a higher level. 'auto' - copy KVM setting. As during live migration SMT settings on the source and destination host may differ this requires us to add a migration blocker. Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com> Message-Id: <20191018163908.10246-1-vkuznets@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> |
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