On Mon, 23 May 2011 22:32:28 +0800, Youquan Song said:

> +		no_x2apic_optout [Default Off]
> +			With this option BIOS x2APIC opt-out request will be
> +			ignored.

> +	else if (!x2apic_supported() && cpu_has_x2apic)
> +		WARN(1, "Your BIOS is broken and requested that x2apic be "
> +			"disabled.\n This will leave your machine vulnerable to"
> +			" irq-injection attacks\n"
> +			"Use 'intel_iommu=no_x2apic_optout' to override BIOS "
> +			"request\n");

If we're doing a WARN level here, what are the downsides of just automagically
forcing it rather than making them use a kernel parameter and reboot?  Will
some systems fail to boot because the BIOS was in fact right in requesting
hat x2apic be turned off?