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

[Xin Li <xin@zytor.com>]

On 6/17/2025 2:03 AM, Peter Zijlstra wrote:
> 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

Yeah, I think it makes sense to make the change.

Thanks!
     Xin