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

From: Vitaly Kuznetsov
Subject: Re: [PATCH RFC] i386/kvm: fix enlightened VMCS with fine-grained VMX feature enablement
Date: Tue, 07 Jan 2020 19:15:40 +0100

Paolo Bonzini <address@hidden> 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:


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.


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