Messages in this thread | | | From | "Zhang, Lin-Bao (Linux Kernel R&D)" <> | Subject | RE: Why Linux kernel forced to enter X2APIC mode( just because of booting cpu has supported x2apic) without depending on BIOS' setting in MSR->x2apic enablement bit ? | Date | Wed, 19 Dec 2012 10:13:50 +0000 |
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Hi Yinghai , Thanks very much for your reply. I also checked some other previous emails about x2apic patches.
> From: yhlu.kernel@gmail.com [mailto:yhlu.kernel@gmail.com] On Behalf Of > Yinghai Lu
I just worry about this case, > > 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 ? 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 .
> 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 ?
Thanks very much !
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