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Re: [PATCH 2/3] x86/defconfig: add CONFIG_IRQ_REMAP
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Shashank Balaji <shashank.mahadasyam@xxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, x86@xxxxxxxxxx, "H. Peter Anvin" <hpa@xxxxxxxxx>, Suresh Siddha <suresh.b.siddha@xxxxxxxxx>, "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>, Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>, Wei Liu <wei.liu@xxxxxxxxxx>, Dexuan Cui <decui@xxxxxxxxxxxxx>, Long Li <longli@xxxxxxxxxxxxx>, Ajay Kaher <ajay.kaher@xxxxxxxxxxxx>, Alexey Makhalov <alexey.makhalov@xxxxxxxxxxxx>, Broadcom internal kernel review list <bcm-kernel-feedback-list@xxxxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
- From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
- Date: Mon, 2 Feb 2026 12:54:30 +0100
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- Cc: Ingo Molnar <mingo@xxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, linux-hyperv@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxx, jailhouse-dev@xxxxxxxxxxxxxxxx, kvm@xxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx, Rahul Bukte <rahul.bukte@xxxxxxxx>, Daniel Palmer <daniel.palmer@xxxxxxxx>, Tim Bird <tim.bird@xxxxxxxx>
- Delivery-date: Mon, 02 Feb 2026 11:54:56 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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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