linux-kernel - Re: [PATCH v4] x86, vt-d: enable x2apic opt out

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Date:	Thu, 26 May 2011 00:01:19 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Youquan Song <youquan.song@...el.com>,
	linux-kernel@...r.kernel.org, david.woodhouse@...el.com,
	akpm@...ux-foundation.org, hpa@...or.com, hpa@...ux.intel.com,
	allen.m.kay@...el.com, suresh.b.siddha@...el.com,
	rajesh.sankaran@...el.com, asit.k.mallick@...el.com,
	kent.liu@...el.com, Youquan Song <youquan.song@...ux.intel.com>
Subject: Re: [PATCH v4] x86, vt-d: enable x2apic opt out

On Wed, 25 May 2011, Ingo Molnar wrote:

> 
> * Youquan Song <youquan.song@...el.com> wrote:
> 
> > New version of VT-d2 specification 
> > (http://download.intel.com/technology 
> > /computing/vptech/Intel(r)_VT_for_Direct_IO.pdf) includes a new 
> > feature that provide firmware a way to request system software to 
> > opt out of enable x2APIC mode. DMAR ACPI table newly define flags.1 
> > bit: x2APIC_OPT_OUT which is set to request System software opt out 
> > xAPIC mode if flags.0 bit:INTR_REMAP is also set.
> 
> So why isnt the x2apic disabled in the CPUID? That's the canonical 
> way to unsupport a particular non-working CPU hw feature.

Because some committee decided to make it an ACPI feature. That's
broken by design, but you can't change the stupid spec retroactively.

Thanks,

	tglx
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