Re: [Xen-devel] APIC MSRs query

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/05/11 14:43, Jan Beulich wrote:
> > > > On 17.05.11 at 15:25, Andrew Cooper<andrew.cooper3@xxxxxxxxxx>  wrote:
> > 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?
> I think this is a benign mistake. Benign because I don't think there is
> a meaningful (to Xen at least) number of systems that would not
> have their LAPIC at the default address (which fits in 32 bits).
>
> Jan
Ok - I will fix this up in my cleanup.

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


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