[PATCH 0/3] KVM: x86: hyper-v: make L2 Hyper-V 2019 on KVM guests see MD_CLEAR

[Vitaly Kuznetsov]

[The series is KVM specific but the first patch of the series likely requires
 someone else's ACK. hyperv-tlfs.h gets a small addition too.]

It was discovered that L2 guests on Hyper-V 2019 on KVM don't see MD_CLEAR
bit (and thus think they're MDS vulnerable) even when it is present on the
host. Turns out, Hyper-V is filtering it out because it is not sure the
topology L0 is exposing is trustworthy and generally it is not. In some
specific cases (e.g. when SMT is unsupported or forcesully disabled) it is
and we can tell this to userspace hoping that it'll pass this info to L1.
See PATCH2 of the series for additional details.

The series can be tested with QEMU-4.1+ and 'hv-passthrough' CPU flag.

Vitaly Kuznetsov (3):
  cpu/SMT: create and export cpu_smt_possible()
  KVM: x86: hyper-v: set NoNonArchitecturalCoreSharing CPUID bit when
    SMT is impossible
  KVM: selftests: hyperv_cpuid: add check for
    NoNonArchitecturalCoreSharing bit

 arch/x86/include/asm/hyperv-tlfs.h            |  7 +++++
 arch/x86/kvm/hyperv.c                         |  4 ++-
 include/linux/cpu.h                           |  2 ++
 kernel/cpu.c                                  | 11 ++++++--
 .../selftests/kvm/x86_64/hyperv_cpuid.c       | 27 +++++++++++++++++++
 5 files changed, 48 insertions(+), 3 deletions(-)

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2.20.1