Re: [PATCH] arm_mpam: resctrl: Catch and propagate error from get_cpu_cacheinfo_id()

[Andre Przywara <andre.przywara@arm.com>]

Hi,

On 9/16/26 11:49, Ben Horgan wrote:
> Hi Andre,
> 
> On 07/09/2026 13:59, Ben Horgan wrote:
>> Hi Andre,
>>
>> On 02/09/2026 15:37, Andre Przywara wrote:
>>> get_cpu_cacheinfo_id() can fail, in which case it returns a negative
>>> error value.
>>>
>>> Check the returned value for this error condition, before passing the
>>> value on to other code, which would hide the negative number in some high
>>> value in the unsigned type.
>>>
>>> Fixes: 36528c7681b8 ("arm_mpam: resctrl: Add support for 'MB' resource")
>>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>>
>> This looks good to me. Out of interest what led you to find this?
>>
>> Reviewed-by: Ben Horgan <ben.horgan@arm.com>
> 
> I seem to have been a bit hasty here.
> 
> Sashiko points out at [1] that 0xFFFFFFFF is the only value we were previously considering invalid
> and that the value coming from dt or acpi can provid other valid values that would after this patch
> be considered invalid.

Fair, and I can easily change the check to only check explicitly for -1.

But this is somewhat broken already, right? I mean the return type for 
the existing get_cpu_cacheinfo_id() has always been "int". It looks like 
this comes from the x86 world, where the cache IDs never get that large? 
I can have a deeper look, but my gut feeling is that this is practically 
irrelevant, since we barely see Aff3 at all, not to mention high values 
of it.

Cheers,
Andre

> 
> [1] https://sashiko.dev/#/patchset/20260902143757.3469690-1-andre.przywara%40arm.com
> 
> Thanks,
> 
> Ben>
>> Thanks,
>>
>> Ben
>>
>>> ---
>>>   drivers/resctrl/mpam_resctrl.c | 5 ++++-
>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
>>> index 9d223057953ab..a5e661eff86d7 100644
>>> --- a/drivers/resctrl/mpam_resctrl.c
>>> +++ b/drivers/resctrl/mpam_resctrl.c
>>> @@ -786,7 +786,10 @@ static u32 get_mba_min(struct mpam_props *cprops)
>>>   /* Find the L3 cache that has affinity with this CPU */
>>>   static int find_l3_equivalent_bitmask(int cpu, cpumask_var_t tmp_cpumask)
>>>   {
>>> -	u32 cache_id = get_cpu_cacheinfo_id(cpu, 3);
>>> +	int cache_id = get_cpu_cacheinfo_id(cpu, 3);
>>> +
>>> +	if (cache_id < 0)
>>> +		return -ENOENT;
>>>   
>>>   	lockdep_assert_cpus_held();
>>>   
>>
>>
>