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

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Message-ID: <Z9vZYIbXOz2wF59j@gmail.com>
Date: Thu, 20 Mar 2025 10:01:20 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Balbir Singh <balbirs@...dia.com>
Cc: Bert Karwatzki <spasswolf@....de>, Alex Deucher <alexdeucher@...il.com>,
	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


* Balbir Singh <balbirs@...dia.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?

Thanks,

	Ingo