linux-kernel - Re: CXL Boot to Bash - Section 3: Memory (block) Hotplug

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Message-ID: <Z7eLNyB230-wYof-@gourry-fedora-PF4VCD3F>
Date: Thu, 20 Feb 2025 15:06:15 -0500
From: Gregory Price <gourry@...rry.net>
To: David Hildenbrand <david@...hat.com>
Cc: Yang Shi <shy828301@...il.com>, lsf-pc@...ts.linux-foundation.org,
	linux-mm@...ck.org, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: CXL Boot to Bash - Section 3: Memory (block) Hotplug

On Thu, Feb 20, 2025 at 08:44:36PM +0100, David Hildenbrand wrote:
> On 20.02.25 20:35, Gregory Price wrote:
> 
> There were some discussions around variable-sized memory blocks. But we
> cannot rip out the old stuff, because it breaks API and thereby Linux user
> space. There would have to be a world switch (e.g., kernel cmdline, config
> option)
> 
> ... and some corner cases are rather nasty (e.g., hotunplugging boot memory,
> where we only learn from ACPI which regions actually belong to a single
> DIMM; dlpar hotunplugging individual memory blocks of boot memory IIRC).
> 

Probably we just end back up between a rock and a hard place then.  Let
me finish this documentation series and cleaning up corrections and I'll
come back around on this and figure out if it's feasible.

If variable sized memory blocks in infeasible, then the only solution to
misaligned memory regions seems to be to wag a finger at hardware vendors
as Dan suggests.

Thinking about my current acpi/block size extension, it does seem bad
that the user can't choose to force a larger block sizes without a
boot parameter - but i hesitate to add *yet another* switch on top of
that. The problem is complicated enough.

~Gregory