linux-kernel - Re: [PATCH] KVM: nVMX: Dynamically compute max VMCS index for vmcs12

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Date:   Tue, 6 Jul 2021 07:42:46 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Hu, Robert" <robert.hu@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: nVMX: Dynamically compute max VMCS index for vmcs12

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