linux-kernel - Re: [PATCH 1/1] x86/mm/pae: Align up pteval_t, pmdval_t and pudval

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Date: Thu, 4 Apr 2024 11:31:58 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Javier Pello <devel@...eo.eu>, linux-kernel@...r.kernel.org,
 x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/1] x86/mm/pae: Align up pteval_t, pmdval_t and pudval_t
 to avoid split locks

On 4/4/24 08:26, Sean Christopherson wrote:
> In other words, practically speaking this isn't about supporting a new hardware
> feature on 32-bit kernels, it's about preserving performance in real world
> scenarios when running 32-bit kernels on new hardware.

Realistically, most of the 32-bit kernels in the world are going to be
*OLD* distros, right?  Old CentOS/RHEL/SLES kernels from before the
kernel had split lock detection, or split lock fixes.  Those trip over
VMM split lock detection now, and presumably will forever.

I suspect new CentOS/RHEL/SLES kernels that have split lock detection
all happened after 32-bit support was dropped from those distros.

I think that basically leaves Debian.  Someone would need to:

 1. Make a *new* 32-bit Debian distro install (or one of the other
    less common distros that still do 32-bit)
 2. Run it on hardware with split lock detection
 3. On a VMM that enables split lock detection
 4. Stay close enough to mainline to get split lock fixes (like from
    this thread)
 5. Care about performance, despite *ACTIVELY* choosing a 32-bit distro
    on 64-bit hardware in 2024

Those steps are certainly possible.  I'm just not sure how much trouble
we want to go to in 2024 to support people that choose new 32-bit
distros and desire performance.  It feels to me to be approaching "I
want a pony" territory.

Or am I just lacking empathy today? :)