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Subject: Re: [PATCH] KVM: nVMX: Dynamically compute max VMCS index for vmcs12
To:     "Hu, Robert" <robert.hu@intel.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20210618214658.2700765-1-seanjc@google.com>
 <c847e00a-e422-cdc9-3317-fbbd82b6e418@redhat.com>
 <YNDHfX0cntj72sk6@google.com> <da6c715345954a7b91c044ad685eb0f2@intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <d98092db-b277-44f5-df5e-f367ecb5a0fc@redhat.com>
Date:   Tue, 6 Jul 2021 07:42:46 +0200
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On 06/07/21 05:05, Hu, Robert wrote:
>> As noted in the code comments, KVM allows VMREAD/VMWRITE to all defined
>> fields, whether or not the field should actually exist for the vCPU model doesn't
>> enter into the equation.  That's technically wrong as there are a number of
>> fields that the SDM explicitly states exist iff a certain feature is supported.  To
>> fix that we'd need to add a "feature flag" to vmcs_field_to_offset_table that is
>> checked against the vCPU model, though updating the MSR would probably fall
>> onto userspace's shoulders?
> 
> [Hu, Robert]
> Perhaps more easier and proper to do this in KVM side.
> QEMU sets actual feature set down to KVM, and KVM updates IA32_VMX_VMCS_ENUM
> MSR accordingly. We don't see a channel that QEMU constructs a VMCS and sets a whole
> to KVM.

Yes, it's possible to do that too.  If that is included in Linux 5.14, 
we can remove it from QEMU.

Paolo