linux-kernel - Re: [PATCH v2] x86, vt-d: enable x2apic opt out

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Date:	Wed, 18 May 2011 23:58:57 +0100
From:	"Woodhouse, David" <david.woodhouse@...el.com>
To:	Alex Williamson <alex.williamson@...hat.com>
CC:	"Song, Youquan" <youquan.song@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	"Kay, Allen M" <allen.m.kay@...el.com>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"Liu, Kent" <kent.liu@...el.com>,
	Youquan Song <youquan.song@...ux.intel.com>
Subject: Re: [PATCH v2] x86, vt-d: enable x2apic opt out

On Mon, 2011-05-16 at 21:32 +0100, Alex Williamson wrote:
> 
> > Is this just a workaround for a crappy BIOS? What is the *actual* reason
> > for wanting to disable x2apic?
> 
> Just a guess, but the OEM probably hasn't updated their SMI handlers to
> understand x2apic yet and won't before the product ships because some
> other OS doesn't bother to use x2apic. 

But I think we'll still use x2apic if interrupt remapping isn't enabled
(for example if VT-d is disabled in the BIOS and the whole DMAR table is
absent), or if we're booted with 'iommu=off' or indeed if the
distribution vendor has hacked the kernel so that iommu=off is the
*default*, because they've seen so many broken BIOSes.

AFAICT although CONFIG_X86_X2APIC depends on CONFIG_INTR_REMAP, we can
still enable x2apic at runtime if interrupt remapping is not operating?
We end up hitting this code in enable_IR_x2apic():

	if (!ret) {
		/* IR is required if there is APIC ID > 255 even when running
		 * under KVM
		 */
		if (max_physical_apicid > 255 ||
		    !hypervisor_x2apic_available())
			goto nox2apic;
		/*
		 * without IR all CPUs can be addressed by IOAPIC/MSI
		 * only in physical mode
		 */
		x2apic_force_phys();
	}

So if that *is* the reason, this doesn't seem like a viable solution.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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