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

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2022-02-23 at 14:10 +0800, Chao Gao wrote:
> On Thu, Feb 03, 2022 at 08:22:13PM +0000, Sean Christopherson wrote:
> > i.e. ACPI_NUMA gets priority and thus amd_numa_init() will never be reached if
> > the NUMA topology is enumerated in the ACPI tables.  Furthermore, the VMM would
> > have to actually emulate an old AMD northbridge, which is also extremely unlikely.
> > 
> > The odds of breaking a guest are further diminised given that KVM doesn't emulate
> > the xAPIC ID => x2APIC ID hilarity on AMD CPUs and no one has complained.
> > 
> > So, rather than tie this to IPI virtualization, I think we should either make
> > the xAPIC ID read-only across the board,
> 
> We will go this way and defer the introduction of "xapic_id_writable" to the
> emergence of the "crazy" use case.
> 
> Levitsky, we plan to revise your patch 13 "[PATCH RESEND 13/30] KVM: x86: lapic:
> don't allow to change APIC ID when apic acceleration is enabled" to make xAPIC
> ID read-only regardless of APICv/AVIC and include it into IPI virtualization
> series (to eliminate the dependency on your AVIC series). Is it fine with you?


Absolutely!
> And does this patch 13 depend on other patches in your fixes?

This patch doesn't depend on anything.

There is also patch 14 in this series which closes a case where malicious userspace
could upload non default _x2apic id_. I  haven't yet written a unit test
to demonstrate this, but I will soon.

You don't need that patch for now IMHO.

> 
> > or if we want to hedge in case someone
> > has a crazy use case, make the xAPIC ID read-only by default, add a module param
> > to let userspace opt-in to a writable xAPIC ID, and report x2APIC and APICv as
> > unsupported if the xAPIC ID is writable.  E.g. rougly this, plus your AVIC patches
> > if we want to hedge.


Best regards,
	Maxim Levitsky