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 -- 2.17.1 Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879
