Re: [PATCH v4 0/3] Allow user space to restrict and augment MSR emulation

[Jim Mattson]

On Mon, Aug 3, 2020 at 2:14 PM Alexander Graf <graf@xxxxxxxxxx> wrote:
>
> 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.

This is somewhat misleading. If you don't modify the MSR permission
bitmaps, as Aaron has done, you cannot emulate *arbitrary* MSRs in
userspace. You can only emulate MSRs that kvm is going to intercept.
Moreover, since the set of intercepted MSRs evolves over time, this
isn't a stable API.

> v1 -> v2:
>
>   - s/ETRAP_TO_USER_SPACE/ENOENT/g
>   - 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
>   - s/KVM_EXIT_RDMSR/KVM_EXIT_X86_RDMSR/g
>   - s/KVM_EXIT_WRMSR/KVM_EXIT_X86_WRMSR/g
>   - 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
>   - s/KVM_CAP_ADD_MSR_ALLOWLIST/KVM_CAP_X86_MSR_ALLOWLIST/g
>   - Add KVM_X86_CLEAR_MSR_ALLOWLIST
>   - 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
>
> 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                            | 259 +++++++++++++++++-
>  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, 692 insertions(+), 9 deletions(-)
>  create mode 100644 tools/testing/selftests/kvm/x86_64/user_msr_test.c
>
> --
> 2.17.1
>
>
>
>
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