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

From: Conor Dooley <conor@kernel.org>
To: "Christoph Müllner" <christoph.muellner@vrull.eu>
Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Philipp Tomsich" <philipp.tomsich@vrull.eu>,
	"Björn Töpel" <bjorn@kernel.org>,
	"Daniel Henrique Barboza" <dbarboza@ventanamicro.com>,
	"Heiko Stuebner" <heiko@sntech.de>,
	"Cooper Qu" <cooper.qu@linux.alibaba.com>,
	"Zhiwei Liu" <zhiwei_liu@linux.alibaba.com>,
	"Huang Tao" <eric.huang@linux.alibaba.com>,
	"Alistair Francis" <alistair.francis@wdc.com>,
	"Andrew Jones" <ajones@ventanamicro.com>,
	"Qingfang Deng" <dqfext@gmail.com>,
	"Alexandre Ghiti" <alex@ghiti.fr>
Subject: Re: [PATCH v2 2/2] riscv: T-Head: Test availability bit before enabling MAEE errata
Date: Thu, 4 Apr 2024 17:02:57 +0100	[thread overview]
Message-ID: <20240404-drown-wronged-bfc421e412aa@spud> (raw)
In-Reply-To: <20240329121414.688391-3-christoph.muellner@vrull.eu>

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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@vrull.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@microchip.com>

Thanks,
Conor.

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