From: Jan Kiszka <jan.kiszka@siemens.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>,
Shashank Balaji <shashank.mahadasyam@sony.com>,
Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
Suresh Siddha <suresh.b.siddha@intel.com>,
"K. Y. Srinivasan" <kys@microsoft.com>,
Haiyang Zhang <haiyangz@microsoft.com>,
Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
Long Li <longli@microsoft.com>,
Ajay Kaher <ajay.kaher@broadcom.com>,
Alexey Makhalov <alexey.makhalov@broadcom.com>,
Broadcom internal kernel review list
<bcm-kernel-feedback-list@broadcom.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Juergen Gross <jgross@suse.com>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Ingo Molnar <mingo@elte.hu>,
linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
virtualization@lists.linux.dev, jailhouse-dev@googlegroups.com,
kvm@vger.kernel.org, xen-devel@lists.xenproject.org,
Rahul Bukte <rahul.bukte@sony.com>,
Daniel Palmer <daniel.palmer@sony.com>,
Tim Bird <tim.bird@sony.com>
Subject: Re: [PATCH 2/3] x86/defconfig: add CONFIG_IRQ_REMAP
Date: Mon, 2 Feb 2026 12:54:30 +0100 [thread overview]
Message-ID: <f875676e-c878-4d3e-9eae-1f74f24cdedd@siemens.com> (raw)
In-Reply-To: <a7d93306-42e5-4617-91df-23f7dd35aa1c@citrix.com>
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
--
Siemens AG, Foundational Technologies
Linux Expert Center
next prev parent reply other threads:[~2026-02-02 11:54 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-02-02 9:51 [PATCH 0/3] x86/x2apic: Fix hang-up of defconfig kernel on resume from s2ram Shashank Balaji
2026-02-02 9:51 ` [PATCH 1/3] x86/x2apic: disable x2apic on resume if the kernel expects so Shashank Balaji
2026-02-02 15:02 ` kernel test robot
2026-02-02 22:31 ` kernel test robot
2026-02-03 0:24 ` Shashank Balaji
2026-02-03 21:08 ` Sohil Mehta
2026-02-04 9:17 ` Shashank Balaji
2026-02-04 18:53 ` Sohil Mehta
2026-02-05 6:07 ` Shashank Balaji
2026-02-05 23:18 ` Sohil Mehta
2026-02-06 3:44 ` Shashank Balaji
2026-02-06 8:57 ` Shashank Balaji
2026-02-07 0:37 ` Sohil Mehta
2026-02-02 9:51 ` [PATCH 2/3] x86/defconfig: add CONFIG_IRQ_REMAP Shashank Balaji
2026-02-02 11:35 ` Andrew Cooper
2026-02-02 11:54 ` Jan Kiszka [this message]
2026-02-02 12:12 ` Andrew Cooper
2026-02-02 13:50 ` Jan Kiszka
2026-02-02 9:51 ` [PATCH 3/3] x86/virt: rename x2apic_available to x2apic_without_ir_available Shashank Balaji
2026-02-06 0:10 ` Sohil Mehta
2026-02-06 9:23 ` Shashank Balaji
2026-02-13 7:39 ` Shashank Balaji
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