[v2,0/3] Allow user space to restrict and augment MSR emulation

Message

Alexander Graf July 29, 2020, 11:59 p.m. UTC

While tying to add support for the MSR_CORE_THREAD_COUNT MSR in KVM,
I realized that we were still in a world where user space has no control
over what happens with MSR emulation in KVM.

That is bad for multiple reasons. In my case, I wanted to emulate the
MSR in user space, because it's a CPU specific register that does not
exist on older CPUs and that really only contains informational data that
is on the package level, so it's a natural fit for user space to provide
it.

However, it is also bad on a platform compatibility level. Currrently,
KVM has no way to expose different MSRs based on the selected target CPU
type.

This patch set introduces a way for user space to indicate to KVM which
MSRs should be handled in kernel space. With that, we can solve part of
the platform compatibility story. Or at least we can not handle AMD specific
MSRs on an Intel platform and vice versa.

In addition, it introduces a way for user space to get into the loop
when an MSR access would generate a #GP fault, such as when KVM finds an
MSR that is not handled by the in-kernel MSR emulation or when the guest
is trying to access reserved registers.

In combination with the allow list, the user space trapping allows us
to emulate arbitrary MSRs in user space, paving the way for target CPU
specific MSR implementations from user space.

Alexander Graf (3):
  KVM: x86: Deflect unknown MSR accesses to user space
  KVM: x86: Introduce allow list for MSR emulation
  KVM: selftests: Add test for user space MSR handling

 Documentation/virt/kvm/api.rst                | 115 +++++++++
 arch/x86/include/asm/kvm_host.h               |  13 +
 arch/x86/include/uapi/asm/kvm.h               |  15 ++
 arch/x86/kvm/emulate.c                        |  18 +-
 arch/x86/kvm/x86.c                            | 234 +++++++++++++++++-
 include/trace/events/kvm.h                    |   2 +-
 include/uapi/linux/kvm.h                      |  15 ++
 tools/testing/selftests/kvm/Makefile          |   1 +
 .../selftests/kvm/x86_64/user_msr_test.c      | 199 +++++++++++++++
 9 files changed, 605 insertions(+), 7 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/x86_64/user_msr_test.c