Re: [PATCH] kvm/x86/vmx: switch MSR_MISC_FEATURES_ENABLES between host and guest

From: Paolo Bonzini
Date: Thu Mar 14 2019 - 10:44:48 EST

On 14/03/19 13:37, Xiaoyao Li wrote:
>> Adding a RDMSR for this to each vmentry is too heavy. Since we emulate
>> MSR_MISC_FEATURES_ENABLES, you can just clear the MSR on vcpu_load and
>> restore it on vcpu_put.
> One question here. Just clear the MSR on vcpu_load instead of writing the
> emulated value to MSR?
> I think writing the emulated value to MSR is better. As I mentioned in case 3,
> if hardware has cpuid faulting feature. Using hardware capability is more
> efficient than emulation that the emulation solution needs VM exit to inject
> #GP.

You can do that too, yes. You can add it to vmx_msr_index and it will
be handled like that.

However, that wouldn't work on AMD (which doesn't use the
kvm_define_shared_msr infrastructure) and also on hosts that don't have
MSR_MISC_FEATURES_ENABLES, so my suggestion is to add this optimization
as a separate patch.