On 12/13/22 07:23, Sean Christopherson wrote:
Don't modify the set of allowed secondary execution controls, i.e. the virtual MSR_IA32_VMX_PROCBASED_CTLS2, in response to guest CPUID changes. To avoid breaking old userspace that never sets the VMX MSRs, i.e. relies on KVM to provide a consistent vCPU model, keep the existing behavior if userspace has never written MSR_IA32_VMX_PROCBASED_CTLS2. KVM should not modify the VMX capabilities presented to L1 based on CPUID as doing so may discard explicit settings provided by userspace. E.g. if userspace does KVM_SET_MSRS => KVM_SET_CPUID and disables a feature in the VMX MSRs but not CPUID (to prevent exposing the feature to L2), then stuffing the VMX MSRs during KVM_SET_CPUID will expose the feature to L2 against userspace's wishes.
The commit message doesn't explain *why* KVM_SET_CPUID would be done before KVM_SET_MSRS. The presence of certain MSRs or bits within is signaled by CPUID bits, and even though KVM is more lenient on host-initiated MSR writes it still verifies in general that the bits are valid.
For now I applied patch 1 and (with a reworded comment) patch 2. I'm not opposed to the rest, but I would like to better understand the reason for them. (If it has been reported to the mailing list, please add a "Link" trailer too).
Paolo
