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Re: [PATCH 1/3] x86/x2apic: disable x2apic on resume if the kernel expects so
- To: Shashank Balaji <shashank.mahadasyam@xxxxxxxx>
- From: Sohil Mehta <sohil.mehta@xxxxxxxxx>
- Date: Fri, 6 Feb 2026 16:37:26 -0800
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- Cc: 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>, Jan Kiszka <jan.kiszka@xxxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, 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>, <stable@xxxxxxxxxxxxxxx>
- Delivery-date: Sat, 07 Feb 2026 00:38:11 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 2/6/2026 12:57 AM, Shashank Balaji wrote:
> On Thu, Feb 05, 2026 at 03:18:58PM -0800, Sohil Mehta wrote:
>> On 2/4/2026 10:07 PM, Shashank Balaji wrote:
>>> On Wed, Feb 04, 2026 at 10:53:28AM -0800, Sohil Mehta wrote:
>>
>>>> It's a bit odd then that the firmware chooses to enable x2apic without
>>>> the OS requesting it.
>>>
>>> Well, the firmware has a setting saying "Enable x2apic", which was
>>> enabled. So it did what the setting says
>>>
>>
I still think the firmware behavior is flawed. Some confusion
originates from the option "Enable x2apic". Based on just these two
words, the behavior seems highly ambiguous. I interpret it to mean "Make
the x2apic feature available to the kernel". Then it is up to the kernel
whether it decides to use it.
If the intention of the BIOS option is to automatically/always enable
x2apic then there is an architectural way to do it. It can set the bits
that track to x2apic_hw_locked(). But, doing it this way during resume
(behind OS's back) is unexpected.
>>
>> pr_warn_once("x2apic unexpectedly re-enabled by the firmware during
>> resume.\n");
>
> At least as per the spec, it's not something the firmware needs to fix,
> and it's not unexpected re-enablement.
>
> Am I missing something?
>
> But it _is_ surprising that this bug went unnoticed for so long :)
Maybe this BIOS option isn't typically found in a production firmware. I
think it would be preferable to have the pr_warn_once() but I won't push
super hard for it due to the unclear information.
Whatever you choose for v2, can you please briefly describe the
ambiguity in the commit message? It would help other reviewers provide
better insight and be handy if we ever need to come back to this.
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