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

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Message-ID: <aYMOqXTYMJ_IlEFA@JPC00244420>
Date: Wed, 4 Feb 2026 18:17:29 +0900
From: Shashank Balaji <shashank.mahadasyam@...y.com>
To: Sohil Mehta <sohil.mehta@...el.com>
Cc: Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Long Li <longli@...rosoft.com>,
	Ajay Kaher <ajay.kaher@...adcom.com>,
	Alexey Makhalov <alexey.makhalov@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	Juergen Gross <jgross@...e.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-hyperv@...r.kernel.org, virtualization@...ts.linux.dev,
	jailhouse-dev@...glegroups.com, kvm@...r.kernel.org,
	xen-devel@...ts.xenproject.org, Rahul Bukte <rahul.bukte@...y.com>,
	Daniel Palmer <daniel.palmer@...y.com>,
	Tim Bird <tim.bird@...y.com>, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] x86/x2apic: disable x2apic on resume if the kernel
 expects so

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/