On Wed, Dec 19, 2012 at 2:13 AM, Zhang, Lin-Bao (Linux Kernel R&D) <[email protected]> wrote: >> > b) If BIOS feel the system doesn't meet x2apic conditions , it will not set >> x2apic enablement bit in MSR ,and pass control to OS with xapic mode. >> >> kernel will check if cpuid support x2apic, if it supports x2apic, it will >> check if >> DMAR/intr-remapping could be enabled, if so kernel will switch to x2apic. >> otherwise it will stay with xapic. >> >> So you need to make cpuid show does not support x2apic --- check with intel >> they have way to do that. > My concerns is BIOS creates different ACPI tables in xAPIC mode and X2APIC > mode. > BIOS will auto detect if the system meets x2APIC condition , if no , it will > not create right ACPI tables for this mode and don't set MSR bit. > For this case (BIOS didn't set MSR , but BSP supports x2apic ) , I want to > know > 1, For Linux kernel, which ACPI tables will be used for x2apic and XAPIC ?
two MADT? why two? according to Intel, if your apic id < 255, even cpu is in x2apic state already, you only have xapic entry in MADT. > 2, why Linux kernel will set x2apic enablement bit in CPU msr ? generally > speaking , BIOS set it and OS get it. > If OS get it , who will read it ? BIOS ? I consult BIOS designer , he said , > their BIOS calling service(just like UEFI run time service) > will also judge MSR again , so I don't know why Linux kernel will set MSR bit > , unless OS has special target . because os does not trust BIOS. BIOS ONLY SHOULD only do thing it should do: 1. initialize memory. 2. assign some resource to pci devices bar 3. provide memory mem-map to bootloader and os. If it don't want OS to use x2apic, disable that in CPUID. > >> or you can pass "nox2apic" in boot command line. > Yeah, I think this would be fine if we really want to use xAPIC mode instead > of x2apic mode. > But I am puzzled why Linux kernel will force to enter x2apic just depends on > BSP has supported x2apic ? > "BSP supports x2apic" condition is enough for judging if OS can enable x2apic > ? if system support x2apic, os will try use x2apic at first, unless BIOS mess it up. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
