Re: [PATCH 3/9] KVM: x86: SVM: Pass through shadow stack MSRs

[Sean Christopherson]

On Tue, Nov 07, 2023, Maxim Levitsky wrote:
> Since no sane L1 hypervisor will ever allow access to all its msrs from L2,
> it might make sense to always use a dedicated MSR bitmap for L2.

Hmm, there might be a full passthrough use case out there, but in general, yeah,
I agree.  I think even kernel hardening use cases where the "hypervisor" is just
a lowvisor would utilize MSR bitmaps to prevent modifying the de-privileged
kernel from modifying select MSRs.

> Also since all sane L1 hypervisors do use a msr bitmap means that
> dedicated code path that doesn't use it is not well tested.
> 
> On VMX if I am not mistaken, this is not an issue because either all
> MSRS are intercepted or a bitmap is used.

Yep, if the MSR bitmaps aren't used then all MSR accesses are intercepted.