linux-kernel - Re: [PATCH v2 2/2] riscv: T-Head: Test availability bit before enabling MAEE errata

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Date: Thu, 4 Apr 2024 17:02:57 +0100
From: Conor Dooley <conor@...nel.org>
To: Christoph Müllner <christoph.muellner@...ll.eu>
Cc: 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>,
	Qingfang Deng <dqfext@...il.com>, Alexandre Ghiti <alex@...ti.fr>
Subject: Re: [PATCH v2 2/2] riscv: T-Head: Test availability bit before
 enabling MAEE errata

On Fri, Mar 29, 2024 at 01:14:14PM +0100, Christoph Müllner wrote:
> T-Head's MAEE mechanism (non-compatible equivalent of RVI's Svpbmt)
> is currently assumed for all T-Head harts. However, QEMU recently
> decided to drop acceptance of guests that write reserved bits in PTEs.
> As MAEE uses reserved bits in PTEs and Linux applies the MAEE errata
> for all T-Head harts, this broke the Linux startup on QEMU emulations
> of the C906 emulation.
> 
> This patch attempts to address this issue by testing the MAEE bit
> in the th.sxstatus CSR. This CSR is available in HW and can be
> emulated in QEMU.
> 
> This patch also makes the MAEE probing mechanism reliable, because
> a test for the right combination of mvendorid, marchid, and mimpid
> is not sufficient to enable MAEE.
> 
> Signed-off-by: Christoph Müllner <christoph.muellner@...ll.eu>

Seems ideal to me, I'm guessing the QEMU guys were okay with emulating
the CSR. I don't see any screaming at the very least on the patch for
it.
Reviewed-by: Conor Dooley <conor.dooley@...rochip.com>

Thanks,
Conor.

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