Re: [PATCH RFC] i386/kvm: fix enlightened VMCS with fine-grained VMX feature enablement

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 07/01/20 13:08, Vitaly Kuznetsov wrote:
>> Honestly I forgot the story why we filtered out these features upon
>> eVMCS enablement in KVM. As there are no corresponding eVMCS fields,
>> there's no way a guest can actually use them.
>
> Well, mostly because we mimicked what Hyper-V was doing I guess.
>

An update from reverse-engineering trenches.

I ran some tests to see if we can just drop the filtering and there is
only one problematic control which Hyper-V enables:

SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES

the problem with it is that we don't have 'apic_access_addr' field in
eVMCS ('virtual_apic_page_addr' is there). By running the same setup
with eVMCS disabled I figured out which address can be hardcoded to make
it boot. My guess was that the fields is present but not documented
properly, I tried scanning eVMCS for the value but with no luck so far.

I'll try to fish some information out of Microsoft.

-- 
Vitaly