Re: [RFC PATCH v3 03/19] KVM: x86: SVM: remove avic's broken code that updated APIC ID

[Sean Christopherson <seanjc@google.com>]

On Sun, May 22, 2022, Maxim Levitsky wrote:
> On Thu, 2022-05-19 at 16:10 +0000, Sean Christopherson wrote:
> > On Wed, Apr 27, 2022, Maxim Levitsky wrote:
> > > AVIC is now inhibited if the guest changes apic id, thus remove
> > > that broken code.
> > 
> > Can you explicitly call out what's broken?  Just something short on the code not
> > handling the scenario where APIC ID is changed back to vcpu_id to help future
> > archaeologists.  I forget if there are other bugs...
> > 
> 
> 
> Well, the avic_handle_apic_id_update is called each time the AVIC is uninhibited,
> because while it is inhibited, the AVIC code doesn't track changes to APIC ID and such.
> 
> Also there are many ways it is broken for example
> 
> 1. a CPU can't move its APIC ID to a free slot due to (!new) check
> 
> 2. If APIC ID is moved to a used slot, then the CPU that used that overwritten
> slot can't correctly move it, since its now not its slot, not to mention races.

The more the merrier :-)  Any/all of those examples are great, just so long as it's
obvious to future readers that the code truly is busted.

> BTW, if you see a value in it, I can fix this code instead - a lock + going over all the apic ids,
> should be quite easy to implement. In case of two vCPUs using the same APIC ID,
> I can write non present entry to the table, so none will be able to be addressed,
> hoping that the situation is only temporary.

Very strong "no", let's keep this as simple as possible without outright killing
the guest or breaking ABI.  Disabling APICv/AVIC is perfect.