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

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Message-ID: <20250220163043.000000cd@huawei.com>
Date: Thu, 20 Feb 2025 16:30:43 +0000
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Gregory Price <gourry@...rry.net>
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


> 
> Example CEDT Entries (truncated) 
>          Subtable Type : 00 [CXL Host Bridge Structure]
>               Reserved : 00
>                 Length : 0020
> Associated host bridge : 00000005
> 
>          Subtable Type : 01 [CXL Fixed Memory Window Structure]
>               Reserved : 00
>                 Length : 002C
>               Reserved : 00000000
>    Window base address : 000000C050000000
>            Window size : 0000003CA0000000
> 
> If this memory is NOT marked "Special Purpose" by BIOS (next section),

Specific purpose.  You don't want to know how long that term took to
agree on...

> you should find a matching entry EFI Memory Map and /proc/iomem
> 
> BIOS-e820:   [mem 0x000000c050000000-0x000000fcefffffff] usable

Trivial but that's not the EFI memory map, that's the e820.
On some architectures this really will be coming from the EFI memory map.


> /proc/iomem: c050000000-fcefffffff : System RAM


> 
> 
> ---------------------------------------------------------------
> Step 3: EFI_MEMORY_SP - Deferring Management to the CXL Driver.
> ---------------------------------------------------------------
> 
> Assuming you DON'T want CXL memory to default to SystemRAM and prefer
> NOT to have your kernel allocate arbitrary resources on CXL, you
> probably want to defer managing these memory regions to the CXL driver.
> 
> The mechanism for is setting EFI_MEMORY_SP bit on CXL memory in BIOS.
> This will mark the memory "Special Purpose".

Specific purpose if we are keeping the UEFI naming.