[RFC PATCH 02/36] arm64: debug: don't mask DAIF for mdscr_write()

[Vladimir Murzin]

On 7/9/26 14:06, Jinjie Ruan wrote:
> 
> On 7/9/2026 8:12 PM, Vladimir Murzin wrote:
>> From: Ada Couprie Diaz <ada.coupriediaz at arm.com>
>>
>> Masking DAIF around the write to MDSCR_EL1 doesn't do anything: we can
>> write to sysregs with interrupts unmasked, and writing to PSTATE is
>> not a context synchronization event so it does not synchronize it.
>>
>> This is done in the context of a general interrupt handling cleanup,
>> so it does not address the missing context synchronization for the
>> MDSCR_EL1 write, staying consistent with the current state.  This
>> should be addressed in a future patch.
>>
>> Signed-off-by: Ada Couprie Diaz <ada.coupriediaz at arm.com>
>> Signed-off-by: Vladimir Murzin <vladimir.murzin at arm.com>
>> ---
>>  arch/arm64/kernel/debug-monitors.c | 3 ---
>>  1 file changed, 3 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
>> index 29307642f4c9..e271fbac5f82 100644
>> --- a/arch/arm64/kernel/debug-monitors.c
>> +++ b/arch/arm64/kernel/debug-monitors.c
>> @@ -40,10 +40,7 @@ u8 debug_monitors_arch(void)
>>   */
>>  static void mdscr_write(u64 mdscr)
>>  {
>> -	unsigned long flags;
>> -	flags = local_daif_save();
>>  	write_sysreg(mdscr, mdscr_el1);
>> -	local_daif_restore(flags);
> commit 478fcb2cdb23 ("arm64: Debugging support") used the
> local_dbg_save() to set the DAIF.D bit.
> 
> In my opinion, this was originally intended to avoid the debug exception
> being triggered immediately when the single-step mode is enabled during
> the write of MDSCR_EL1.SS?
> 

I'm not sure I follow. The scenario you described, specifically the
phrase "during the write", suggests that the system register can be
partially updated. Is that what you meant?

If you meant "after the write", then perhaps I'm missing some
important detail about how

msr	mdscr_el1, [val]
<exception>

differs from

msr	mdscr_el1, [val]
msr	daifclr, #8
<exception>

Thanks
Vladimir


> 
> + * MDSCR access routines.
> + */
> +static void mdscr_write(u32 mdscr)
> +{
> +       unsigned long flags;
> +       local_dbg_save(flags);
> +       asm volatile("msr mdscr_el1, %0" :: "r" (mdscr));
> +       local_dbg_restore(flags);
> +}
> 
> 
> +#define local_dbg_save(flags)
>        \
> +       do {
>        \
> +               typecheck(unsigned long, flags);
>        \
> +               asm volatile(
>        \
> +               "mrs    %0, daif                        //
> local_dbg_save\n"    \
> +               "msr    daifset, #8"
>        \
> +               : "=r" (flags) : : "memory");
>        \
> +       } while (0)
> 
> 
>>  }
>>  NOKPROBE_SYMBOL(mdscr_write);
>>  
>