On 3/20/25 20:01, Ingo Molnar wrote:
>
> * Balbir Singh <balb...@nvidia.com> wrote:
>
>> On 3/17/25 00:09, Bert Karwatzki wrote:
>>> This is related to the admgpu.gttsize. My laptop has the maximum amount
>>> of memory (64G) and usually gttsize is half of main memory size. I just
>>> tested with cmdline="nokaslr amdgpi.gttsize=2048" and the problem does
>>> not occur. So I did some more testing with varying gttsize and got this
>>> for the built-in GPU
>>>
>>> 08:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI]
>>> Cezanne [Radeon Vega Series / Radeon Vega Mobile Series] (rev c5)
>>>
>>> (nokaslr is always enabeld)
>>> gttssize input behaviour
>>> 2048 GOOD
>>> 2064 GOOD
>>> 2080 SEMIBAD (i.e. noticeable input lag but not as bad as
>>> below)
>>> 3072 BAD
>>> 4096 BAD
>>> 8192 BAD
>>> 16384 BAD
>>>
>>> As the build-in GPU has ~512 VRAM there seems to be problems when gttsize >
>>> 4*VRAM so I tested for the discrete GPU with 8G of VRAM
>>> gttsize input behaviour
>>> 49152 GOOD
>>> 64000 GOOD
>>>
>>> So for the discrete GPU increasing gttsize does no reproduce the bug.
>>>
>>
>> Very interesting, I am not a GTT expert, but with these experiments do you
>> find anything interesting in
>>
>> /sys/kernel/debug/x86/pat_memtype_list?
>>
>> It's weird that you don't see any issues in Xorg (Xfce), just the games.
>> May be we should get help from the amd-gfx experts to further diagnose/debug
>> the interaction of nokaslr with the game.
>
> So basically your commit:
>
> 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
>
> inflicts part of the effects of a 'nokaslr' boot command line option,
> and triggers the regression due to that?
>
> Or is there some other cause?
>
You are right in your assessment of the root cause. Just to reiterate
- nokaslr does not work with the iGPU, specifically for the games mentioned
- There is a workaround for the problem, which involves reducing the
amdgpu.gttsize
- The patch exposes the system to nokaslr situation (effect) when PCI_P2PDMA is
enabled
Balbir Singh
