Messages in this thread | | | Date | Fri, 11 Feb 2022 07:10:30 +0700 | Subject | Re: [PATCH v5] KVM: SVM: Allow AVIC support on system w/ physical APIC ID > 255 | From | "Suthikulpanit, Suravee" <> |
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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
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