Re: [Xen-devel] [PATCH] x86: always store APIC ID of CPU

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.06.14 at 13:02, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 26/06/14 11:11, Jan Beulich wrote:
>> +/*
>> + * cpuid returns the value latched in the HW at reset, not the APIC ID
>> + * register's value.  For any box whose BIOS changes APIC IDs, like
>> + * clustered APIC systems, we must use hard_smp_processor_id.
>> + *
>> + * See Intel's IA-32 SW Dev's Manual Vol2 under CPUID.
>> + */
>> +static inline u32 phys_pkg_id(u32 cpuid_apic, int index_msb)
> 
> What is the point of the cpuid_apic parameter?  It is unused.
> 
> index_msb should probably be unsigned as it is used to shift with.

For both of these - yes, I agree, but no, I don't want to change
this here (I'm just moving the function up after all). The history of
this is a bit difficult to interpret: Before in change in 2006 some
variants of this function used both inputs, but some didn't. I
suppose Keir didn't want to risk breaking clustered APIC systems,
albeit as far as I looked only Summit systems were actually ignoring
the passed in APIC ID, and we're not supporting these anymore.

As to shift counts, I don't think it really matters whether they're
signed or unsigned, as anything outside the range [0,bitwidth-1]
produce undefined operations anyway.

Jan


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