linux-kernel - Re: [tip: x86/urgent] x86/apic: Don't access the APIC when disabling X2APIC

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Date: Tue, 30 Apr 2024 07:55:35 +0200
From: Ingo Molnar <mingo@...nel.org>
To: linux-kernel@...r.kernel.org
Cc: linux-tip-commits@...r.kernel.org, Adrian Huang <ahuang12@...ovo.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Borislav Petkov (AMD)" <bp@...en8.de>, stable@...r.kernel.org,
	x86@...nel.org
Subject: Re: [tip: x86/urgent] x86/apic: Don't access the APIC when disabling
 X2APIC


* tip-bot2 for Thomas Gleixner <tip-bot2@...utronix.de> wrote:

> -	apic_set_fixmap();
> +	/*
> +	 * Don't reread the APIC ID as it was already done from
> +	 * check_x2apic() and the apic driver still is a x2APIC variant,
> +	 * which fails to do the read after x2APIC was disabled.
> +	 */
> +	apic_set_fixmap(false);

JFYI, I amended the commit with the fixlet below, to avoid the inevitable 
followup trivial patch:

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index b229648b7a18..803dcfb0e346 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1795,7 +1795,7 @@ static __init void x2apic_disable(void)
 	__x2apic_disable();
 	/*
 	 * Don't reread the APIC ID as it was already done from
-	 * check_x2apic() and the apic driver still is a x2APIC variant,
+	 * check_x2apic() and the APIC driver still is a x2APIC variant,
 	 * which fails to do the read after x2APIC was disabled.
 	 */
 	apic_set_fixmap(false);

Thanks,

	Ingo