How to deal with openSBI reserved regions?

From: Petr Tesarik <petrtesarik@huaweicloud.com>
To: linux-riscv@lists.infradead.org
Subject: How to deal with openSBI reserved regions?
Date: Fri, 28 Jul 2023 12:02:49 +0200	[thread overview]
Message-ID: <76ff0f51-d6c1-580d-f943-061e93073306@huaweicloud.com> (raw)

Hi all,

I have recently looked into enabling crash kernel and kdump on riscv64.
I can start a new kernel after crash, but I ran into an issue when
reading /proc/vmcore there.

I am testing this inside a QEMU VM and I boot my system in S-Mode using
U-Boot with mbedded openSBI firmware. The problem here is that openSBI
occupies the first 128 pages of RAM, but they are shown as "System RAM"
in /proc/iomem. The kexec_file_load(2) system call uses
walk_system_ram_res() to build a memory map of the currently running
kernel. This is then passed to the crash kernel through ELF core headers
as a LOAD segment. When reading the corresponding part of /proc/iomem,
the crash kernel tries to map these pages, but they can be accessed only
from M-Mode. Any attempt to access them from the Linux kernel fails with
a PMP violation. Consequently, -EFAULT is returned to user space.

Now, the openSBI area is represented in the Device Tree with a
reserved-memory node () which overlaps a memory node. Technically, the
firmware region is indeed RAM, but how should it be excluded from
/proc/iomem?

Should this be fixed in the kernel?

Or, is the provided DTB incorrect?
Should the memory node exclude the firmware area?

Petr T

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