linux-kernel - Re: [PATCH 1/4] KVM: nVMX: support restore of VMX capability MSRs

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Message-ID: <CALzav=fPNw1QYHBjxzgwE9r_DnjdRGBRTyiyRWbcT5qEmGsEMg@mail.gmail.com>
Date:   Mon, 28 Nov 2016 14:57:02 -0800
From:   David Matlack <dmatlack@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jim Mattson <jmattson@...gle.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 1/4] KVM: nVMX: support restore of VMX capability MSRs

On Mon, Nov 28, 2016 at 2:48 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> On 28/11/2016 22:11, David Matlack wrote:
>> > PINBASED_CTLS, PROCBASED_CTLS, EXIT_CTLS and ENTRY_CTLS can be derived
>> > from their "true" counterparts, so I think it's better to remove the
>> > "non-true" ones from struct nested_vmx (and/or add the "true" ones when
>> > missing) and make them entirely computed.  But it can be done on top.
>>
>> Good point. And that would mean userspace does not need to restore the
>> non-true MSRs, right?
>
> Yes, sorry for being a bit too concise. :)

I'll include this cleanup in the next version of the patchset since it
affects which MSRs userspace will restore. It looks like a pretty
simple patch.

>
>> KVM does not emulate MSR_IA32_VMX_BASIC[55]=0,
>> and will probably never want to.
>
> That's a separate question, MSR_IA32_VMX_BASIC[55]=0 basically means
> that the "true" capabilities are the same as the "default" capabilities.
>  If userspace wanted to set it that way, KVM right now would not hide
> the "true" capability MSR, but on the other hand the nested hypervisor
> should not even notice the difference.

KVM would also need to use the non-true MSR in place of the true MSRs
when checking VMCS12 during VM-entry.

>
> Paolo