• Vitaly Kuznetsov's avatar
    KVM: x86: hyper-v: set NoNonArchitecturalCoreSharing CPUID bit when SMT is impossible · b2d8b167
    Vitaly Kuznetsov authored
    Hyper-V 2019 doesn't expose MD_CLEAR CPUID bit to guests when it cannot
    guarantee that two virtual processors won't end up running on sibling SMT
    threads without knowing about it. This is done as an optimization as in
    this case there is nothing the guest can do to protect itself against MDS
    and issuing additional flush requests is just pointless. On bare metal the
    topology is known, however, when Hyper-V is running nested (e.g. on top of
    KVM) it needs an additional piece of information: a confirmation that the
    exposed topology (wrt vCPU placement on different SMT threads) is
    trustworthy.
    
    NoNonArchitecturalCoreSharing (CPUID 0x40000004 EAX bit 18) is described in
    TLFS as follows: "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." From KVM we can give such
    guarantee in two cases:
    - SMT is unsupported or forcefully disabled (just 'disabled' doesn't work
     as it can become re-enabled during the lifetime of the guest).
    - vCPUs are properly pinned so the scheduler won't put them on sibling
    SMT threads (when they're not reported as such).
    
    This patch reports NoNonArchitecturalCoreSharing bit in to userspace in the
    first case. The second case is outside of KVM's domain of responsibility
    (as vCPU pinning is actually done by someone who manages KVM's userspace -
    e.g. libvirt pinning QEMU threads).
    Signed-off-by: default avatarVitaly Kuznetsov <vkuznets@redhat.com>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    b2d8b167
hyperv-tlfs.h 24.9 KB