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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 02/02/2026 11:54 am, Jan Kiszka wrote:
> 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)."

This statement is misleading and has been argued over before.  It's
missing the key word "all".

What IR gets you in this case is the ability to target CPU 255 and higher.

The OS-side access mechanism (xAPIC MMIO vs x2APIC MSRs) has no baring
on how external interrupts are handled in the fabric.

There are plenty of good reasons to have Interrupt Remapping enabled
when available, but it is not a hard requirement architecturally.

~Andrew