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

From: Paolo Bonzini <pbonzini@redhat.com>
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>
Subject: Re: [PATCH] KVM: nVMX: Dynamically compute max VMCS index for vmcs12
Date: Tue, 6 Jul 2021 07:42:46 +0200	[thread overview]
Message-ID: <d98092db-b277-44f5-df5e-f367ecb5a0fc@redhat.com> (raw)
In-Reply-To: <da6c715345954a7b91c044ad685eb0f2@intel.com>

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