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Re: [Xen-devel] APIC MSRs query

On 17/05/11 14:59, Ian Campbell wrote:
On Tue, 2011-05-17 at 14:43 +0100, Jan Beulich wrote:
On 17.05.11 at 15:25, Andrew Cooper<andrew.cooper3@xxxxxxxxxx>  wrote:

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 = (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).
That "msr_content = (uint32_t)msr_content;" seems to be pretty
deliberate, what else would it be trying to do?

FWIW enable_x2apic in Linux seems to have a similar construct which
throws away the top half of the MSR:

void enable_x2apic(void)
        int msr, msr2;

        rdmsr(MSR_IA32_APICBASE, msr, msr2);
        if (!(msr&  X2APIC_ENABLE)) {
                printk("Enabling x2apic\n");
                wrmsr(MSR_IA32_APICBASE, msr | X2APIC_ENABLE, 0);

(FWIW the original Xen code in 17545:9fd00ff95068 looked a lot like this
too, b622e411eef8 just switched to wrmsrl and preserved the clearing

Perhaps there is some errata? Google didn't find one, but ...


I couldn't find any errata which is why I asked here. Bits [63:36] are reserved so should be WriteAsZero - it is possible that whoever put it into Linux just wanted to zero the top bits and either missed the top 4 bits or decided that they would never be set.

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

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