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 15:33:05 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, 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()

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?

Paolo