[PATCH 0/3] x86/kvm/hyper-v: fix enlightened VMCS & QEMU4.2

[Vitaly Kuznetsov <vkuznets@redhat.com>]

With fine grained VMX feature enablement QEMU>=4.2 tries to do KVM_SET_MSRS
with default (matching CPU model) values and in case eVMCS is also enabled,
fails. While the regression is in QEMU, it may still be preferable to
fix this in KVM.

It would be great if we could just omit the VMX feature filtering in KVM
and make this guest's responsibility: if it switches to using enlightened
vmcs it should be aware that not all hardware features are going to be
supported. Genuine Hyper-V, however, fails the test. It, for example,
enables SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES and without
'apic_access_addr' field in eVMCS there's not much we can do in KVM.
Microsoft confirms the bug but it is unclear if it will ever get fixed
in the existing Hyper-V versions as genuine Hyper-V never exposes
these unsupported controls to L1.

Changes since 'RFC':
- Go with the bare minimum [Paolo]

KVM RFC:
https://lore.kernel.org/kvm/20200115171014.56405-1-vkuznets@redhat.com/

QEMU RFC@:
https://lists.nongnu.org/archive/html/qemu-devel/2020-01/msg00123.html

Vitaly Kuznetsov (3):
  x86/kvm/hyper-v: remove stale evmcs_already_enabled check from
    nested_enable_evmcs()
  x86/kvm/hyper-v: move VMX controls sanitization out of
    nested_enable_evmcs()
  x86/kvm/hyper-v: don't allow to turn on unsupported VMX controls for
    nested guests

 arch/x86/kvm/vmx/evmcs.c  | 90 ++++++++++++++++++++++++++++++++++-----
 arch/x86/kvm/vmx/evmcs.h  |  3 ++
 arch/x86/kvm/vmx/nested.c |  3 ++
 arch/x86/kvm/vmx/vmx.c    | 16 ++++++-
 4 files changed, 99 insertions(+), 13 deletions(-)

-- 
2.24.1