linux-kernel - Re: x2apic boot failure on recent sandy bridge system

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Date:	Sun, 11 Dec 2011 23:23:22 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Ingo Molnar <mingo@...e.hu>, "Berck E. Nash" <flyboy@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: x2apic boot failure on recent sandy bridge system

On Sun, 2011-12-11 at 13:43 -0800, Yinghai Lu wrote:
> -v4: according to Ingo, for x2apic pre-enabled system, if intr-remap can not
>         be enabled, try to disable x2apic instead of panic and request to
>         specify nox2apic for next boot. 

If bios has not exported any DMAR tables but enabled x2apic pre-boot, it
is tricky for the OS to disable x2apic and continue with xapic for
number of reasons. For example, if chipset has already enabled
interrupt-remapping (which is typically recommended to be enabled before
x2apic) but didn't provide DMAR tables, then there is no way OS can
disable interrupt-remapping in this case.

So even though the cpu might be back in xapic mode, but external
interrupts won't work.

thanks,
suresh

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