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[Xen-devel] APIC MSRs query

To: xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] APIC MSRs query
From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Date: Tue, 17 May 2011 14:25:52 +0100
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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


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