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

[David Matlack]

On Tue, Nov 29, 2016 at 12:01 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>> On Mon, Nov 28, 2016 at 2:48 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> 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.
>
> Don't bother removing the "non-true" registers from nested_vmx; you only
> need to adjust the userspace API.

I already wrote the patch, so unless there's an argument against
removing them I'll include it in the next patchset. Thanks!

>
>> >
>> >> 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.
>
> It's not necessary, userspace would set the relevant bits to 1 in the true
> MSRs, for both the low and high parts.  If it doesn't, it's garbage in
> garbage out.
>
> Paolo