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Subject: Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization
 out of nested_enable_evmcs()
To:     Vitaly Kuznetsov <vkuznets@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>
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>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <f15d9e98-25e9-2031-2db5-6aaa6c78c0eb@redhat.com>
Date:   Wed, 22 Jan 2020 17:40:27 +0100
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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?

Paolo