Re: [PATCH 14/19] KVM: x86: Honor architectural behavior for aliased 8-bit APIC IDs

[Maxim Levitsky]

On Wed, 2022-08-31 at 16:41 +0000, Sean Christopherson wrote:
> On Wed, Aug 31, 2022, Maxim Levitsky wrote:
> > On Wed, 2022-08-31 at 00:35 +0000, Sean Christopherson wrote:
> > > -		if (!apic_x2apic_mode(apic) && !new->phys_map[xapic_id])
> > > -			new->phys_map[xapic_id] = apic;
> > > +		if (kvm->arch.x2apic_format) {
> > > +			/* See also kvm_apic_match_physical_addr(). */
> > > +			if ((apic_x2apic_mode(apic) || x2apic_id > 0xff) &&
> > > +			    x2apic_id <= new->max_apic_id)
> > > +				new->phys_map[x2apic_id] = apic;
> > > +
> > > +			if (!apic_x2apic_mode(apic) && !new->phys_map[xapic_id])
> > > +				new->phys_map[xapic_id] = apic;
> > > +		} else {
> > > +			/*
> > > +			 * Disable the optimized map if the physical APIC ID is
> > > +			 * already mapped, i.e. is aliased to multiple vCPUs.
> > > +			 * The optimized map requires a strict 1:1 mapping
> > > +			 * between IDs and vCPUs.
> > > +			 */
> > > +			if (apic_x2apic_mode(apic))
> > > +				physical_id = x2apic_id;
> > > +			else
> > > +				physical_id = xapic_id;
> > > +
> > > +			if (new->phys_map[physical_id]) {
> > > +				kvfree(new);
> > > +				new = NULL;
> > > +				goto out;
> > Why not to use the same  KVM_APIC_MODE_XAPIC_FLAT |  KVM_APIC_MODE_XAPIC_CLUSTER
> > hack here?
> 
> The map's "mode" only covers logical mode (the cleanup patch renames "mode" to
> "logical_mode" to make this more clear).  There is no equivalent for dealing with
> the physical IDs.  Alternatively, a flag to say "physical map is disabled" could
> be added, but KVM already has to cleanly handle a NULL map and in all likelihood
> the logical map is also going to be disabled anyways.
> 
> Not to mention that APIC performance is unlikely to be a priority for any guest
> that triggers this code :-)
> 

Thanks for the explanation, that makes sense!

Best regards,
	Maxim Levitsky