linux-kernel - Re: [LSF/MM] CXL Boot to Bash - Section 1: BIOS, EFI, and Early Boot

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Message-ID: <Z871B9WVRaeIYuOs@gourry-fedora-PF4VCD3F>
Date: Mon, 10 Mar 2025 10:19:51 -0400
From: Gregory Price <gourry@...rry.net>
To: Yuquan Wang <wangyuquan1236@...tium.com.cn>
Cc: lsf-pc@...ts.linux-foundation.org, linux-mm@...ck.org,
	linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [LSF/MM] CXL Boot to Bash - Section 1: BIOS, EFI, and Early Boot

On Mon, Mar 10, 2025 at 06:45:12PM +0800, Yuquan Wang wrote:
> On Tue, Feb 04, 2025 at 09:17:09PM -0500, Gregory Price wrote:
> > 
> > Platform / BIOS / EFI Configuraiton
> > ===================================
> > ---------------------------------------
> > Step 1: BIOS-time hardware programming.
> > ---------------------------------------
> > 
> > I don't want to focus on platform specifics, so really all you need
> > to know about this phase for the purpose of MM is that platforms may
> > program the CXL device heirarchy and lock the configuration.
> > 
> This question can be very naive, what's the meaning of 'MM' here?
> 

Memory Management - linux/mm

> And since I am not familiar with cxl bios configurations, based on my
> understanding of its acpi results, there are roughly two configuration
> schemes in my analysis: a) users should enter some configuration
> information manually (like region base/size). b) bios could provide a
> recommendatory configuration by device information.
> 

The BIOS must produce ACPI tables to set aside system physical memory
address space.  *How* BIOS produces these ACPI tables (CEDT + SRAT vs
CEDT only) dictates whether this configuration is static or dynamic.

The devices will provide a CDAT (coherent device attribute table) used
by BIOS to generate these ACPI tables.

All of this dictates how linux configures its NUMA topology, programs
CXL HDM decoders, and how it associates device physical memory with numa
nodes and such.

~Gregory