linux-kernel - Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization out of nested_enable

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Date:   Wed, 22 Jan 2020 16:08:55 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        linux-kernel@...r.kernel.org, Liran Alon <liran.alon@...cle.com>,
        Roman Kagan <rkagan@...tuozzo.com>
Subject: Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization out of nested_enable_evmcs()

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 22/01/20 06:47, Sean Christopherson wrote:
>>> Yes, it most likely is and it would be nice if Microsoft fixed it, but I
>>> guess we're stuck with it for existing Windows versions.  Well, for one
>>> we found a bug in Hyper-V and not the converse. :)
>>>
>>> There is a problem with this approach, in that we're stuck with it
>>> forever due to live migration.  But I guess if in the future eVMCS v2
>>> adds an apic_address field we can limit the hack to eVMCS v1.  Another
>>> possibility is to use the quirks mechanism but it's overkill for now.
>>>
>>> Unless there are objections, I plan to apply these patches.
>> Doesn't applying this patch contradict your earlier opinion?  This patch
>> would still hide the affected controls from the guest because the host
>> controls enlightened_vmcs_enabled.
>
> It does.  Unfortunately the key sentence is "we're stuck with it for
> existing Windows versions". :(
>
>> Rather than update vmx->nested.msrs or filter vmx_get_msr(), what about
>> manually adding eVMCS consistency checks on the disallowed bits and handle
>> SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES as a one-off case by simply
>> clearing it from the eVMCS?  Or alternatively, squashing all the disallowed
>> bits.
>
> Hmm, that is also a possibility.  It's a very hacky one, but I guess
> adding APIC virtualization to eVMCS would require bumping the version to
> 2.  Vitaly, what do you think?

As I already replied to Sean I like the idea to filter out unsupported
controls from eVMCS but unfortunately it doesn't work: Hyper-V actually
expects APIC virtualization to work when it enables
SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES (I have no idea how without
apic_access_addr field but). I checked and at least Hyper-V 2016 doesn't
boot (when >1 vCPU).

-- 
Vitaly