On 09/11/2016 18:15, Igor Mammedov wrote:
> > When IA32_APIC_BASE[11] is 0, the processor is functionally equivalent
> > to an IA-32 processor without an on-chip APIC. The CPUID feature flag
> > for the APIC (see Section 10.4.2, "Presence of the Local APIC") is
> > also set to 0.
>
> CCing Eduardo in case it might affect migration.
If the destination kernel is old, it will re-grow the CPUID APIC bit.
However, clearing IA32_APIC_BASE[11] should really only happen in the
BIOS; our firmware is sane and doesn't do that.
See this bit in arch/x86/kernel/apic/apic.c's apic_force_enable:
rdmsr(MSR_IA32_APICBASE, l, h);
if (!(l & MSR_IA32_APICBASE_ENABLE)) {
pr_info("Local APIC disabled by BIOS -- reenabling.\n");
l &= ~MSR_IA32_APICBASE_BASE;
l |= MSR_IA32_APICBASE_ENABLE | addr;
wrmsr(MSR_IA32_APICBASE, l, h);
enabled_via_apicbase = 1;
}
Paolo
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