linux-kernel - [PATCH 0/2] RISC-V: Test th.mxstatus.MAEE bit before enabling MAEE

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Date: Wed, 27 Mar 2024 11:31:28 +0100
From: Christoph Müllner <christoph.muellner@...ll.eu>
To: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Palmer Dabbelt <palmer@...belt.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Philipp Tomsich <philipp.tomsich@...ll.eu>,
	Björn Töpel <bjorn@...nel.org>,
	Daniel Henrique Barboza <dbarboza@...tanamicro.com>,
	Heiko Stuebner <heiko@...ech.de>,
	Cooper Qu <cooper.qu@...ux.alibaba.com>,
	Zhiwei Liu <zhiwei_liu@...ux.alibaba.com>,
	Huang Tao <eric.huang@...ux.alibaba.com>,
	Alistair Francis <alistair.francis@....com>,
	Andrew Jones <ajones@...tanamicro.com>,
	Conor Dooley <conor@...nel.org>
Cc: Christoph Müllner <christoph.muellner@...ll.eu>
Subject: [PATCH 0/2] RISC-V: Test th.mxstatus.MAEE bit before enabling MAEE

Currently, the Linux kernel suffers from a boot regression when running
on the c906 QEMU emulation. Details have been reported here by Björn Töpel:
  https://lists.gnu.org/archive/html/qemu-devel/2024-01/msg04766.html

The main issue is, that Linux enables MAEE for CPUs that have a T-Head
mvendorid but QEMU maintainers don't want to emulate a CPU that uses
reserved bits in PTEs. See also the following discussion for more
context:
  https://lists.gnu.org/archive/html/qemu-devel/2024-02/msg00775.html

This series renames "T-Head PBMT" to "MAEE" and only enables it if
the th.mxstatus.MAEE bit is set.

Since th.mxstatus is only accessible in M-mode, this patch depends on
the following OpenSBI series:
  http://lists.infradead.org/pipermail/opensbi/2024-March/006718.html

The th.mxstatus CSR is documented here:
  https://github.com/T-head-Semi/thead-extension-spec/pull/45

The QEMU patch to emulate th.mxstatus with MAEE not set is here:
  https://lists.gnu.org/archive/html/qemu-devel/2024-03/msg06368.html

After applying all referenced patchsets, this patchset allows to
successfully boot a C906 QEMU system emulation ("-cpu thead-c906").

Christoph Müllner (2):
  riscv: thead: Rename T-Head PBMT to MAEE
  riscv: T-Head: Test availability bit before enabling MAEE errata

 arch/riscv/Kconfig.errata            |  8 ++++----
 arch/riscv/errata/thead/errata.c     | 22 ++++++++++++++--------
 arch/riscv/include/asm/errata_list.h | 20 ++++++++++----------
 3 files changed, 28 insertions(+), 22 deletions(-)

-- 
2.44.0