linux-kernel - Re: [PATCH v2 1/2] KVM: VMX: Don't advertise EPT switching if EPT itself is not exposed

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Date:   Tue, 17 Oct 2017 19:35:48 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>, Wanpeng Li <kernellwp@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v2 1/2] KVM: VMX: Don't advertise EPT switching if EPT
 itself is not exposed

On 17/10/2017 19:29, Jim Mattson wrote:
> Following the same line of reasoning, what if
> vmx->nested.nested_vmx_secondary_ctls_high is 0 after clearing
> SECONDARY_EXEC_ENABLE_VMFUNC? Does it make sense to report
> CPU_BASED_ACTIVATE_SECONDARY_CONTROLS if we don't actually support any
> of the secondary controls?

All-zero is a valid value for secondary controls, so I think yes.  Besides:

1) userspace can always get into a situation where there are no valid
secondary controls but processor-based execution controls have bit 31 as
1-allowed;

2) I doubt that vmfunc can be the one bit that causes
nested_vmx_secondary_ctls_high to become zero :)

Paolo