linux-kernel - Re: [PATCH v2 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

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Message-ID: <20250617090329.GO1613376@noisy.programming.kicks-ass.net>
Date: Tue, 17 Jun 2025 11:03:29 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Xin Li (Intel)" <xin@...or.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, tglx@...utronix.de,
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
	x86@...nel.org, hpa@...or.com, seanjc@...gle.com,
	pbonzini@...hat.com, sohil.mehta@...el.com, brgerst@...il.com,
	tony.luck@...el.com, fenghuay@...dia.com
Subject: Re: [PATCH v2 1/2] x86/traps: Initialize DR6 by writing its
 architectural reset value

On Tue, Jun 17, 2025 at 12:32:33AM -0700, Xin Li (Intel) wrote:
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 8feb8fd2957a..3bd7c9ac7576 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2243,20 +2243,17 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>  #endif
>  #endif
>  
> -/*
> - * Clear all 6 debug registers:
> - */
> -static void clear_all_debug_regs(void)
> +static void initialize_debug_regs(void)
>  {
>  	int i;
>  
> -	for (i = 0; i < 8; i++) {
> -		/* Ignore db4, db5 */
> -		if ((i == 4) || (i == 5))
> -			continue;
> +	/* Control register first */
> +	set_debugreg(0, 7);
> +	set_debugreg(DR6_RESERVED, 6);
>  
> +	/* Ignore db4, db5 */
> +	for (i = 0; i < 4; i++)
>  		set_debugreg(0, i);
> -	}
>  }
>  
>  #ifdef CONFIG_KGDB

Maybe like so?

--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2206,15 +2206,14 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard)
 
 static void initialize_debug_regs(void)
 {
-	int i;
-
-	/* Control register first */
+	/* Control register first -- to make sure everything is disabled. */
 	set_debugreg(0, 7);
 	set_debugreg(DR6_RESERVED, 6);
-
-	/* Ignore db4, db5 */
-	for (i = 0; i < 4; i++)
-		set_debugreg(0, i);
+	/* dr5 and dr4 don't exist */
+	set_debugreg(0, 3);
+	set_debugreg(0, 2);
+	set_debugreg(0, 1);
+	set_debugreg(0, 0);
 }
 
 #ifdef CONFIG_KGDB