Re: [PATCH 2/3] x86/defconfig: add CONFIG_IRQ_REMAP

[Jan Kiszka]

On 02.02.26 12:35, Andrew Cooper wrote:
> On 02/02/2026 9:51 am, Shashank Balaji wrote:
>> Interrupt remapping is an architectural dependency of x2apic, which is already
>> enabled in the defconfig.
> 
> There is no such dependency.  VMs for example commonly have x2APIC and
> no IOMMU, and even native system with fewer than 254 CPUs does not need
> interrupt remapping for IO-APIC interrupts to function correctly.
> 

It is theoretically possible with less than 254 CPUs, and that is why
virtualization uses it, but the Intel SDM clearly states:

"Routing of device interrupts to local APIC units operating in x2APIC
mode requires use of the interrupt-remapping architecture specified in
the Intel® Virtualization Technology for Directed I/O (Revision 1.3
and/or later versions)."

IIRC, ignoring this would move us into undefined behaviour space that
may work on some system and did not on others.

Jan

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