[PATCH v5 0/3] Allow user space to restrict and augment MSR emulation

From: Alexander Graf
Date: Fri Aug 07 2020 - 11:57:07 EST

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

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

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.

v1 -> v2:

- deflect all #GP injection events to user space, not just unknown MSRs.
That was we can also deflect allowlist errors later
- fix emulator case
- new patch: KVM: x86: Introduce allow list for MSR emulation
- new patch: KVM: selftests: Add test for user space MSR handling

v2 -> v3:

- return r if r == X86EMUL_IO_NEEDED
- Use complete_userspace_io logic instead of reply field
- Simplify trapping code
- document flags for KVM_X86_ADD_MSR_ALLOWLIST
- generalize exit path, always unlock when returning
- Add test to clear whitelist
- Adjust to reply-less API
- Fix asserts
- Actually trap on MSR_IA32_POWER_CTL writes

v3 -> v4:

- Mention exit reasons in re-enter mandatory section of API documentation
- Clear padding bytes
- Generalize get/set deflect functions
- Remove redundant pending_user_msr field
- lock allow check and clearing
- free bitmaps on clear

v4 -> v5:

- use srcu

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 | 157 ++++++++++-
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 | 266 +++++++++++++++++-
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 | 221 +++++++++++++++
9 files changed, 699 insertions(+), 9 deletions(-)
create mode 100644 tools/testing/selftests/kvm/x86_64/user_msr_test.c


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