linux-kernel - Re: [PATCH v3 1/7] x86/cpu: Use a new feature flag for 5 level paging

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Message-ID: <7c315a0d-1508-4310-b584-ecaeaba52296@amd.com>
Date: Thu, 15 May 2025 23:50:17 +0530
From: Shivank Garg <shivankg@....com>
To: Borislav Petkov <bp@...en8.de>, Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
 Ard Biesheuvel <ardb@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Brian Gerst <brgerst@...il.com>, "Rao, Bharata Bhasker" <bharata@....com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/7] x86/cpu: Use a new feature flag for 5 level paging

On 5/15/2025 6:41 PM, Borislav Petkov wrote:
> On Wed, May 14, 2025 at 12:42:44PM +0200, Ard Biesheuvel wrote:
>> From: Ard Biesheuvel <ardb@...nel.org>
>>
>> Currently, the LA57 CPU feature flag is taken to mean two different
>> things at once:
>> - whether the CPU implements the LA57 extension, and is therefore
>>   capable of supporting 5 level paging;
>> - whether 5 level paging is currently in use.
> 
> Btw, that gunk:
> 
> We had started simplifying the whole 5-level crap:
> 
> https://lore.kernel.org/all/20240621164406.256314-1-kirill.shutemov@linux.intel.com/
> 
> Shivank, I hear the performance issues got resolved in the meantime?
> 
> Thx.
> 

Hi Boris,

I've re-tested the performance concerns we discussed earlier regarding 5-level paging.
Recent tests on a current kernel don't show any performance issues:

AMD EPYC Zen 5 (SMT enabled).
Linux HEAD 6.15.0-rc6+ 088d13246a46

lmbench/lat_pagefault:
numactl --membind=1 --cpunodebind=1 bin/x86_64-linux-gnu/lat_pagefault -N 100 1GB_randomfile

Output values (50 runs, Mean, 2.5 percentile and 97.5 percentile, in microseconds):

4-level (no5lvl option)
Mean: 0.138876
     2.5%     97.5%
0.1384988 0.1392532

4-level (CONFIG_X86_5LEVEL=n)
Mean: 0.137958
     2.5%     97.5%
0.1376473 0.1382687

5-level
Mean: 0.138904
     2.5%     97.5%
0.1384789 0.1393291

After repeating the experiments a few times, the observed difference(~1%) in mean values
is under noise levels.
I think these results address the performance concerns previously raised[1]. I don't
foresee any issues in proceeding with the 5-level paging implementation
simplification efforts[2].

[1] https://lore.kernel.org/all/80734605-1926-4ac7-9c63-006fe3ea6b6a@amd.com
[2] https://lore.kernel.org/all/20240621164406.256314-1-kirill.shutemov@linux.intel.com

Thanks,
Shivank