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 11:30:14 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        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

Similarly, it is legal for the IA32_VMX_VMFUNC MSR to report all-zero.

For consistency, perhaps we should not clear the "enable VM functions"
capability in the IA32_VMX_PROCBASED_CTLS2 MSR just because we do not
support any VM functions.

On Tue, Oct 17, 2017 at 10:35 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 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