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

[Shashank Balaji]

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>

---

Changes in v2:
- Add __x2apic_disable() stub for !CONFIG_X86_X2APIC
- Mention the ACPI spec in the commit message (Sohil Mehta)
---
 arch/x86/kernel/apic/apic.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index d93f87f29d03..c8cb82d5131c 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1894,6 +1894,7 @@ void __init check_x2apic(void)
 
 static inline void try_to_enable_x2apic(int remap_mode) { }
 static inline void __x2apic_enable(void) { }
+static inline void __x2apic_disable(void) { }
 #endif /* !CONFIG_X86_X2APIC */
 
 void __init enable_IR_x2apic(void)
@@ -2456,6 +2457,15 @@ static void lapic_resume(void *data)
 	if (x2apic_mode) {
 		__x2apic_enable();
 	} else {
+		/*
+		 * x2apic may have been re-enabled by the firmware on resuming
+		 * from s2ram
+		 */
+		if (x2apic_enabled()) {
+			pr_warn_once("x2apic: re-enabled by firmware during resume. Disabling\n");
+			__x2apic_disable();
+		}
+
 		/*
 		 * Make sure the APICBASE points to the right address
 		 *

-- 
2.43.0