linux-kernel - Re: commit 7ffb791423c7 breaks steam game

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Message-ID: <b5d80ef2-fd5a-41cc-9184-6c82226c330a@nvidia.com>
Date: Mon, 17 Mar 2025 20:13:10 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Bert Karwatzki <spasswolf@....de>, Alex Deucher <alexdeucher@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>, Kees Cook <kees@...nel.org>,
 Bjorn Helgaas <bhelgaas@...gle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Andy Lutomirski <luto@...nel.org>,
 linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org
Subject: Re: commit 7ffb791423c7 breaks steam game

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.

Balbir