Re: [PATCH v2 1/2] x86/x2apic: Disable x2apic on resume if the kernel expects so

[Thomas Gleixner]

On Fri, Mar 06 2026 at 14:46, Shashank Balaji wrote:

> When resuming from s2ram, firmware may re-enable x2apic mode, which may have
> been disabled by the kernel during boot either because it doesn't support
> irq remapping or for other reasons. This causes the kernel to continue using
> the xapic interface, while the hardware is in x2apic mode, which causes hangs.
> This happens on defconfig + bare metal + s2ram.
>
> Fix this in lapic_resume() by disabling x2apic if the kernel expects it to be
> disabled, i.e. when x2apic_mode = 0.
>
> The ACPI v6.6 spec, Section 16.3 [1] says firmware restores either the pre-sleep
> configuration or initial boot configuration for each CPU, including MSR state:
>
> 	When executing from the power-on reset vector as a result of waking
> 	from an S2 or S3 sleep state, the platform firmware performs only the
> 	hardware initialization required to restore the system to either the
> 	state the platform was in prior to the initial operating system boot,
> 	or to the pre-sleep configuration state. In multiprocessor systems,
> 	non-boot processors should be placed in the same state as prior to the
> 	initial operating system boot.
>
> 	(further ahead)
>
> 	 If this is an S2 or S3 wake, then the platform runtime firmware
> 	 restores minimum context of the system before jumping to the waking
> 	 vector. This includes:
>
> 	 	CPU configuration. Platform runtime firmware restores the
> 		pre-sleep configuration or initial boot configuration of each
> 		CPU (MSR, MTRR, firmware update, SMBase, and so on). Interrupts
> 		must be disabled (for IA-32 processors, disabled by CLI
> 		instruction).
>
> 		(and other things)
>
> So at least as per the spec, re-enablement of x2apic by the firmware is allowed
> if "x2apic on" is a part of the initial boot configuration.
>
> [1] https://uefi.org/specs/ACPI/6.6/16_Waking_and_Sleeping.html#initialization
>
> Fixes: 6e1cb38a2aef ("x64, x2apic/intr-remap: add x2apic support, including enabling interrupt-remapping")
> Cc: stable@xxxxxxxxxxxxxxx
> Co-developed-by: Rahul Bukte <rahul.bukte@xxxxxxxx>
> Signed-off-by: Rahul Bukte <rahul.bukte@xxxxxxxx>
> Signed-off-by: Shashank Balaji <shashank.mahadasyam@xxxxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxx>