Re: [PATCH 1/3] x86/x2apic: disable x2apic on resume if the kernel expects so

[Shashank Balaji <shashank.mahadasyam@sony.com>]

Hi Sohil,

On Tue, Feb 03, 2026 at 01:08:39PM -0800, Sohil Mehta wrote:
> > diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> > index d93f87f29d03..cc64d61f82cf 100644
> > --- a/arch/x86/kernel/apic/apic.c
> > +++ b/arch/x86/kernel/apic/apic.c
> > @@ -2456,6 +2456,12 @@ 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
> > +		 */
> > +		__x2apic_disable();
> > +
> 
> We should likely only disable x2apic on platforms that support it and
> need the disabling. How about?
> 
> ...
> } else {
> 	/*
> 	 *
> 	 */
> 	if (x2apic_enabled())
> 		__x2apic_disable();

__x2apic_disable disables x2apic only if boot_cpu_has(X86_FEATURE_APIC)
and x2apic is already enabled. x2apic_enabled also does the same checks,
the only difference being, it uses rdmsrq_safe instead of just rdmsrq,
which is what __x2apic_disable uses. The safe version is because of
Boris' suggestion [1]. If that's applicable here as well, then rdmsrq in
__x2apic_disable should be changed to rdmsrq_safe.

> I considered if an error message should be printed along with this. But,
> I am not sure if it can really be called a firmware issue. It's probably
> just that newer CPUs might have started defaulting to x2apic on.
> 
> Can you specify what platform you are encountering this?


I'm not sure it's the CPU defaulting to x2apic on. As per Section
12.12.5.1 of the Intel SDM:

	On coming out of reset, the local APIC unit is enabled and is in
	the xAPIC mode: IA32_APIC_BASE[EN]=1 and IA32_APIC_BASE[EXTD]=0.

So, the CPU should be turning on in xapic mode. In fact, when x2apic is
disabled in the firmware, this problem doesn't happen.

Either way, a pr_warn maybe helpful. How about "x2apic re-enabled by the
firmware during resume. Disabling\n"?

[1] https://lore.kernel.org/all/20150116095927.GA18880@pd.tnic/