linux-kernel - Re: [PATCH] x86/apic: Remove logical destination mode for 64-bit

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Message-ID: <c7b68723-6ecb-42be-a9f8-14d64c696283@heusel.eu>
Date: Fri, 9 Aug 2024 11:37:31 +0200
From: Christian Heusel <christian@...sel.eu>
To: Rob <rob@...endal.co.uk>
Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	regressions@...ts.linux.dev, x86@...nel.org, Joerg Roedel <joro@...tes.org>, 
	Tony Luck <tony.luck@...el.com>, LKML <linux-kernel@...r.kernel.org>, 
	Paul Menzel <pmenzel@...gen.mpg.de>, Lyude Paul <lyude@...hat.com>
Subject: Re: [PATCH] x86/apic: Remove logical destination mode for 64-bit

On 24/07/28 05:51PM, Rob wrote:
> * Thomas Gleixner (tglx@...utronix.de) wrote:
> > Logical destination mode of the local APIC is used for systems with up to
> > 8 CPUs. It has an advantage over physical destination mode as it allows to
> > target multiple CPUs at once with IPIs.
> > 
> > That advantage was definitely worth it when systems with up to 8 CPUs
> > were state of the art for servers and workstations, but that's history.
> > 
> > Aside of that there are systems which fail to work with logical destination
> > mode as the ACPI/DMI quirks show and there are AMD Zen1 systems out there
> > which fail when interrupt remapping is enabled. The latter can be cured by
> > firmware updates, but not all OEMs distribute the required changes.
> > 
> > Physical destination mode is guaranteed to work because it is the only way
> > to get a CPU up and running via the INIT/INIT/STARTUP sequence.
> > 
> > As the number of CPUs keeps increasing, logical destination mode becomes a
> > less used code path so there is no real good reason to keep it around.
> > 
> > Therefore remove logical destination mode support for 64-bit and default to
> > physical destination mode.
> 
> Thanks Chris for applying the patch for me.
> 
> Thomas - The patched kernel boots successfully.  I held off updating the
> BIOS so there can be no ambiguity.
> 
> Thanks,
> 
> Rob

As far as I can tell this patch did not get applied anywhere so far,
right? Or did I miss anything?

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