On Thu, Jan 09, 2020 at 09:19:57PM -0800, Sukadev Bhattiprolu wrote: > Ultravisor disables some CPU features like BHRB, EBB and PMU in > secure virtual machines (SVMs). Skip accessing those registers > in SVMs to avoid getting a Program Interrupt. It would be useful to have more explanation of the rationale for the ultravisor disabling access to those features, and indicate whether this is a temporary restriction or a permanent one. If SVMs are never going to be able to use the PMU then that is a bad thing in my opinion. In other words, the commit message should tell us whether the restriction is just because the ultravisor doesn't yet have code for managing and context-switching the PMU, or if there is there some reason why using the PMU in a SVM will always be prohibited for some security-related reason. Also, the only way that a SVM would be getting into the KVM code that you are patching is if it is trying to do nested virtualization. However, the SVM should already know that it is not able to do nested virtualization because the ultravisor should be intercepting and failing the H_SET_PARTITION_TABLE hypercall. So I think there is no good reason for patching the KVM code like you are doing unless the PMU restriction is permanent and we are intending someday to enable SVMs to have nested guests. Paul.
