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

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Date:   Thu, 16 Jan 2020 09:51:47 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Liran Alon <liran.alon@...cle.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-kernel@...r.kernel.org, Roman Kagan <rkagan@...tuozzo.com>
Subject: Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization out of nested_enable_evmcs()

Liran Alon <liran.alon@...cle.com> writes:

>> On 16 Jan 2020, at 1:27, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
>> 
>> On Wed, Jan 15, 2020 at 06:10:13PM +0100, Vitaly Kuznetsov wrote:
>>> With fine grained VMX feature enablement QEMU>=4.2 tries to do KVM_SET_MSRS
>>> with default (matching CPU model) values and in case eVMCS is also enabled,
>>> fails.
>> 
>> As in, Qemu is blindly throwing values at KVM and complains on failure?
>> That seems like a Qemu bug, especially since Qemu needs to explicitly do
>> KVM_CAP_HYPERV_ENLIGHTENED_VMCS to enable eVMCS.
>
> See: https://patchwork.kernel.org/patch/11316021/
> For more context.

Ya,

while it would certainly be possible to require that userspace takes
into account KVM_CAP_HYPERV_ENLIGHTENED_VMCS (which is an opt-in) when
doing KVM_SET_MSRS there doesn't seem to be an existing (easy) way to
figure out which VMX controls were filtered out after enabling
KVM_CAP_HYPERV_ENLIGHTENED_VMCS: KVM_GET_MSRS returns global
&vmcs_config.nested values for VMX MSRs (vmx_get_msr_feature()).

-- 
Vitaly