APIC MSRs query

[Andrew Cooper <andrew.cooper3@citrix.com>]

Hello,

I am currently cleaning up the APIC code for the sake of 
shutdown/reboot/crashdump and have a query about the (modified for 
brevity) snippet of code:

uint64_t msr_content;
rdmsrl(MSR_IA32_APICBASE, msr_content);
msr_content |= MSR_IA32_APICBASE_ENABLE | MSR_IA32_APICBASE_EXTD;
msr_content = (uint32_t)msr_content;
wrmsrl(MSR_IA32_APICBASE, msr_content);

which is added into apic.c in changeset b622e411eef8, and has propagated 
elsewhere in the codebase during subsequent cleanups etc.

The MP spec and x2apic spec states that bits [35:12] of 
MSR_IA32_APICBASE is the base APIC MMIO address.  Is there reason why 
the code (almost always) clears the top 4 bits, or is it just an 
overlooked mistake?

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com