Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization out of nested_enable_evmcs()

[Paolo Bonzini]

On 22/01/20 17:29, Vitaly Kuznetsov wrote:
> Yes, in case we're back to the idea to filter things out in QEMU we can
> do this. What I don't like is that every other userspace which decides
> to enable eVMCS will have to perform the exact same surgery as in case
> it sets allow_unsupported_controls=0 it'll have to know (hardcode) the
> filtering (or KVM_SET_MSRS will fail) and in case it opts for
> allow_unsupported_controls=1 Windows guests just won't boot without the
> filtering.
> 
> It seems to be 1:1, eVMCSv1 requires the filter.

Yes, that's the point.  It *is* a hack in KVM, but it is generally
preferrable to have an easier API for userspace, if there's only one way
to do it.

Though we could be a bit more "surgical" and only remove
SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES---thus minimizing the impact on
non-eVMCS guests.  Vitaly, can you prepare a v2 that does that and adds
a huge "hack alert" comment that explains the discussion?

Paolo