Re: [PATCH v3 3/3] KVM: SVM: Extend host physical APIC ID field to support more than 8-bit

[Suthikulpanit, Suravee]

On 2/1/2022 7:58 PM, Suthikulpanit, Suravee wrote:
> > Anyways, for the new larger mask, IMO dynamically computing the mask based on what
> > APIC IDs were enumerated to the kernel is pointless.  If the AVIC doesn't support
> > using bits 11:0 to address APIC IDs then KVM is silently hosed no matter what if
> > any APIC ID is >255.
>
> The reason for dynamic mask is to protect the reserved bits, which varies between
> the current platform (i.e 11:8) vs. newer platform (i.e. 11:10), in which
> there is no good way to tell except to check the max_physical_apicid (see below).
>
> > Ideally, there would be a feature flag enumerating the larger AVIC support so we
> > could do:
> >
> >     if (!x2apic_mode || !boot_cpu_has(X86_FEATURE_FANCY_NEW_AVIC))
> >         avic_host_physical_id_mask = GENMASK(7:0);
> >     else
> >         avic_host_physical_id_mask = GENMASK(11:0);
> >
> > but since it sounds like that's not the case, and presumably hardware is smart
> > enough not to assign APIC IDs it can't address, this can simply be
> >
> >     if (!x2apic_mode)
> >         avic_host_physical_id_mask = GENMASK(7:0);
> >     else
> >         avic_host_physical_id_mask = GENMASK(11:0);
> >
> > and patch 01 to add+export apic_get_max_phys_apicid() goes away.
>
> Unfortunately, we do not have the "X86_FEATURE_FANCY_NEW_AVIC" CPUID bit :(
>
> Also, based on the previous comment, we can't use the x2APIC mode in the host
> to determine such condition. Hence, the need for dynamic mask based on
> the max_physical_apicid.

I recheck this part, and it should be safe to assume that AVIC HW can support
upto 8-bit (old platform) vs. 12-bit (new platform) depending on the maximum
host physical APIC ID available on the system.

I'll simplify this in v4.

Regards,
Suravee