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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     kvm@vger.kernel.org, Jim Mattson <jmattson@google.com>,
        linux-kernel@vger.kernel.org, Liran Alon <liran.alon@oracle.com>,
        Roman Kagan <rkagan@virtuozzo.com>
Subject: Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization out of nested_enable_evmcs()
In-Reply-To: <f15d9e98-25e9-2031-2db5-6aaa6c78c0eb@redhat.com>
References: <20200115171014.56405-1-vkuznets@redhat.com> <20200115171014.56405-3-vkuznets@redhat.com> <6c4bdb57-08fb-2c2d-9234-b7efffeb72ed@redhat.com> <20200122054724.GD18513@linux.intel.com> <9c126d75-225b-3b1b-d97a-bcec1f189e02@redhat.com> <87eevrsf3s.fsf@vitty.brq.redhat.com> <20200122155108.GA7201@linux.intel.com> <87blqvsbcy.fsf@vitty.brq.redhat.com> <f15d9e98-25e9-2031-2db5-6aaa6c78c0eb@redhat.com>
Date:   Thu, 23 Jan 2020 10:15:41 +0100
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Paolo Bonzini <pbonzini@redhat.com> writes:

> 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?

Yes, sure. I'd like to do more testing to make sure filtering out
SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES is enough for other Hyper-V
versions too (who knows how many bugs are there :-)

-- 
Vitaly