linux-kernel - Re: 32bit resctrl? (was Re: [PATCH v2] fs/resctrl: fix domid loss precision issue)

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Message-ID: <162f5113-4eb6-dcea-f034-c81b9dc021b6@amd.com>
Date: Fri, 15 Mar 2024 11:17:21 -0500
From: "Moger, Babu" <bmoger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>,
 Rex Nie <rex.nie@...uarmicro.com>, james.morse@....com,
 "x86@...nel.org" <x86@...nel.org>, "Luck, Tony" <tony.luck@...el.com>,
 Babu Moger <babu.moger@....com>, Peter Newman <peternewman@...gle.com>,
 Borislav Petkov <bp@...en8.de>
Cc: fenghua.yu@...el.com, ilpo.jarvinen@...ux.intel.com,
 linux-kernel@...r.kernel.org
Subject: Re: 32bit resctrl? (was Re: [PATCH v2] fs/resctrl: fix domid loss
 precision issue)

On 3/14/2024 10:25 AM, Reinette Chatre wrote:
> +x86 maintainers, Tony, Babu, Peter
>
> Hi Everybody,
>
> On 3/12/2024 12:53 AM, Rex Nie wrote:
>> Below statement from mkdir_mondata_subdir function will loss precision,
>> because it assigns int to 14 bits bitfield.
>> 	priv.u.domid = d->id;
>>
>> On some platforms(e.g.,x86), the max cache_id is the amount of L3 caches,
>> so it is not in the range of 0x3fff. But some platforms use higher
>> cache_id, e.g., arm uses cache_id as locator for cache MSC. This will
>> cause below issue if cache_id > 0x3fff likes:
>> /sys/fs/resctrl/mon_groups/p1/mon_data/mon_L3_1048564 # cat llc_occupancy
>> cat: read error: No such file or directory
>>
>> This is the call trace when cat llc_occupancy:
>> rdtgroup_mondata_show()
>> 	domid = md.u.domid
>> 	d = resctrl_arch_find_domain(r, domid)
>>
>> d is null here because of lossing precision
>>
>> Signed-off-by: Rex Nie <rex.nie@...uarmicro.com>
>> ---
>>   fs/resctrl/internal.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/resctrl/internal.h b/fs/resctrl/internal.h
>> index 7a6f46b4edd0..096317610949 100644
>> --- a/fs/resctrl/internal.h
>> +++ b/fs/resctrl/internal.h
>> @@ -94,7 +94,7 @@ union mon_data_bits {
>>   	struct {
>>   		unsigned int rid		: 10;
>>   		enum resctrl_event_id evtid	: 8;
>> -		unsigned int domid		: 14;
>> +		u32				domid;
>>   	} u;
>>   };
>>   
> resctrl currently supports 32bit builds. Fixing this issue* in this way

I have never bothered about 32bit builds.   Is Intel still testing 32bit 
builds?

> would first require that resctrl (the architecture independent fs part)
> depend on X86_64. Is this a change that everybody will be comfortable with?
>
> (Of course, there are other solutions available to address the issue mentioned
> in this patch that do not require depending on X86_64, but I would like
> to take this moment to understand the sentiment surrounding continuing support
> for 32bit resctrl.)

I am thinking we have bigger problem here.

The structure pointer "union mon_data_bits priv;" is created in stack 
and passed to create mondata directory. We are reading it later again in 
rdtgroup_mondata_show.

How is this pointer valid again?  Shouldn't we use static pointer or 
allocate memory for the pointer?

thanks

Babu

>
> Thank you.
>
> Reinette
>
> * Please note that this is not an urgent fix but instead a preparatory change
>    for future Arm support.
>