Re: [PATCH v5 7/8] KVM: VMX: Update PID-pointer table entry when APIC ID is changed

[Chao Gao]

On Fri, Jan 14, 2022 at 10:17:34AM +0200, Maxim Levitsky wrote:
>On Fri, 2022-01-14 at 10:58 +0800, Chao Gao wrote:
>> On Thu, Jan 13, 2022 at 10:19:21PM +0000, Sean Christopherson wrote:
>> > On Tue, Jan 11, 2022, Maxim Levitsky wrote:
>> > > Both Intel and AMD's PRM also state that changing APIC ID is implementation
>> > > dependent.
>> > >  
>> > > I vote to forbid changing apic id, at least in the case any APIC acceleration
>> > > is used, be that APICv or AVIC.
>> > 
>> > That has my vote as well.  For IPIv in particular there's not much concern with
>> > backwards compability, i.e. we can tie the behavior to enable_ipiv.
>Great!
>> 
>> Hi Sean and Levitsky,
>> 
>> Let's align on the implementation.
>> 
>> To disable changes for xAPIC ID when IPIv/AVIC is enabled:
>> 
>> 1. introduce a variable (forbid_apicid_change) for this behavior in kvm.ko
>> and export it so that kvm-intel, kvm-amd can set it when IPIv/AVIC is
>> enabled. To reduce complexity, this variable is a module level setting.
>> 
>> 2. when guest attempts to change xAPIC ID but it is forbidden, KVM prints
>> a warning on host and injects a #GP to guest.
>> 
>> 3. remove AVIC code that deals with changes to xAPIC ID.
>> 
>
>I have a patch for both, I attached them.

Looks good to me. We will drop this patch and rely on the first attached patch
to forbid guest from changing xAPIC ID.

>I haven't tested either of these patches that much other than a smoke test,
>but I did test all of the guests I  have and none broke in regard to boot.
>
>I will send those patches as part of larger patch series that implements
>nesting for AVIC. I hope to do this next week.

Thanks.