Re: [PATCH] perf/x86/intel/uncore: Fix oops when counting IMC uncore events on some TGL

["Liang, Kan" <kan.liang@linux.intel.com>]

On 5/27/2020 10:51 AM, David Laight wrote:
> From: Liang, Kan
>> Sent: 27 May 2020 15:47
>> On 5/27/2020 8:59 AM, David Laight wrote:
>>> From: kan.liang@linux.intel.com
>>>> Sent: 27 May 2020 13:31
>>>>
>>>> From: Kan Liang <kan.liang@linux.intel.com>
>>>>
>>>> When counting IMC uncore events on some TGL machines, an oops will be
>>>> triggered.
>>>>     [ 393.101262] BUG: unable to handle page fault for address:
>>>>     ffffb45200e15858
>>>>     [ 393.101269] #PF: supervisor read access in kernel mode
>>>>     [ 393.101271] #PF: error_code(0x0000) - not-present page
>>>>
>>>> Current perf uncore driver still use the IMC MAP SIZE inherited from
>>>> SNB, which is 0x6000.
>>>> However, the offset of IMC uncore counters for some TGL machines is
>>>> larger than 0x6000, e.g. 0xd8a0.
>>>>
>>>> Enlarge the IMC MAP SIZE for TGL to 0xe000.
>>>
>>> Replacing one 'random' constant with a different one
>>> doesn't seem like a proper fix.
>>>
>>> Surely the actual bounds of the 'memory' area are properly
>>> defined somewhere.
>>> Or at least should come from a table.
>>>
>>> You also need to verify that the offsets are within the mapped area.
>>> An unexpected offset shouldn't try to access an invalid address.
>>
>> Thanks for the review.
>>
>> I agree that we should add a check before mapping the area to prevent
>> the issue happens again.
>>
>> I think the check should be a generic check for all platforms which try
>> to map an area, not just for TGL. I will submit a separate patch for the
>> check.
> 
> You need a check that the actual access is withing the mapped area.
> So instead of getting an OOPS you get a error.
> 
> This is after you've mapped it.

Sure. Will add a WARN_ONCE() before the actual access.

Thanks,
Kan

> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>