On 2/10/2022 12:54 AM, Sean Christopherson wrote:
E.g.
Expand KVM's mask for the AVIC host physical ID to the full 12 bits defined
by the architecture. The number of bits consumed by hardware is model
specific, e.g. early CPUs ignored bits 11:8, but there is no way for KVM
to enumerate the "true" size. So, KVM must allow using all bits, else it
risks rejecting completely legal x2APIC IDs on newer CPUs.
This means KVM relies on hardware to not assign x2APIC IDs that exceed the
"true" width of the field, but presmuably hardware is smart enough to tie
the width to the max x2APIC ID. KVM also relies on hardware to support at
least 8 bits, as the legacy xAPIC ID is writable by software. But, those
assumptions are unavoidable due to the lack of any way to enumerate the
"true" width.
Note, older versions of the APM state that bits 11:8 are reserved for
legacy xAPIC, but consumed for x2APIC. Revision 3.38 corrected this to
state that bits 11:8 are "should be zero" on older hardware.
This last paragraph is incorrect. The APM revision 3.38 and prior states
Reserved/SBZ for legacy APIC; extension of Host Physical APIC ID when
x2APIC is enabled.
Here, "SBZ" means Should Be Zero.
Therefore, I will remove the last paragraph in V6.
Suravee
