Re: [PATCH v3 1/3] KVM: x86: Deflect unknown MSR accesses to user space

[Jim Mattson]

On Fri, Jul 31, 2020 at 2:50 PM Alexander Graf <graf@xxxxxxxxxx> wrote:
>
> MSRs are weird. Some of them are normal control registers, such as EFER.
> Some however are registers that really are model specific, not very
> interesting to virtualization workloads, and not performance critical.
> Others again are really just windows into package configuration.
>
> Out of these MSRs, only the first category is necessary to implement in
> kernel space. Rarely accessed MSRs, MSRs that should be fine tunes against
> certain CPU models and MSRs that contain information on the package level
> are much better suited for user space to process. However, over time we have
> accumulated a lot of MSRs that are not the first category, but still handled
> by in-kernel KVM code.
>
> This patch adds a generic interface to handle WRMSR and RDMSR from user
> space. With this, any future MSR that is part of the latter categories can
> be handled in user space.
>
> Furthermore, it allows us to replace the existing "ignore_msrs" logic with
> something that applies per-VM rather than on the full system. That way you
> can run productive VMs in parallel to experimental ones where you don't care
> about proper MSR handling.
>
> Signed-off-by: Alexander Graf <graf@xxxxxxxxxx>
>
> ---
>
> 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
>
> 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
> ---
>  Documentation/virt/kvm/api.rst  |  62 +++++++++++++++++++
>  arch/x86/include/asm/kvm_host.h |   6 ++
>  arch/x86/kvm/emulate.c          |  18 +++++-
>  arch/x86/kvm/x86.c              | 106 ++++++++++++++++++++++++++++++--
>  include/trace/events/kvm.h      |   2 +-
>  include/uapi/linux/kvm.h        |  10 +++
>  6 files changed, 197 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 320788f81a05..79c3e2fdfae4 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst

The new exit reasons should probably be mentioned here (around line 4866):

.. note::

      For KVM_EXIT_IO, KVM_EXIT_MMIO, KVM_EXIT_OSI, KVM_EXIT_PAPR and
      KVM_EXIT_EPR the corresponding

operations are complete (and guest state is consistent) only after userspace
has re-entered the kernel with KVM_RUN.  The kernel side will first finish
incomplete operations and then check for pending signals.  Userspace
can re-enter the guest with an unmasked signal pending to complete
pending operations.

Other than that, my remaining comments are all nits. Feel free to ignore them.

> +static int kvm_get_msr_user_space(struct kvm_vcpu *vcpu, u32 index)

Return bool rather than int?

> +{
> +       if (!vcpu->kvm->arch.user_space_msr_enabled)
> +               return 0;
> +
> +       vcpu->run->exit_reason = KVM_EXIT_X86_RDMSR;
> +       vcpu->run->msr.error = 0;

Should we clear 'pad' in case anyone can think of a reason to use this
space to extend the API in the future?

> +       vcpu->run->msr.index = index;
> +       vcpu->arch.pending_user_msr = true;
> +       vcpu->arch.complete_userspace_io = complete_emulated_rdmsr;

complete_userspace_io could perhaps be renamed to
complete_userspace_emulation (in a separate commit).

> +
> +       return 1;
> +}
> +
> +static int kvm_set_msr_user_space(struct kvm_vcpu *vcpu, u32 index, u64 data)

Return bool rather than int?

> +{
> +       if (!vcpu->kvm->arch.user_space_msr_enabled)
> +               return 0;
> +
> +       vcpu->run->exit_reason = KVM_EXIT_X86_WRMSR;
> +       vcpu->run->msr.error = 0;

Same question about 'pad' as above.

> +       vcpu->run->msr.index = index;
> +       vcpu->run->msr.data = data;
> +       vcpu->arch.pending_user_msr = true;
> +       vcpu->arch.complete_userspace_io = complete_emulated_wrmsr;
> +
> +       return 1;
> +}
> +

Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>