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From: Yangyu Chen <cyy@cyyself.name>
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	guoren@kernel.org
Subject: Re: [PATCH v3 2/2] riscv: T-Head: Test availability bit before enabling MAE errata
Date: Mon,  8 Apr 2024 09:58:17 +0800
In-Reply-To: <20240407213236.2121592-3-christoph.muellner@vrull.eu>
References: <20240407213236.2121592-3-christoph.muellner@vrull.eu>
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On 2024/4/8 05:32, Christoph Müllner wrote:
> T-Head's memory attribute extension (XTheadMae) (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 XTheadMae uses reserved bits in PTEs and Linux applies the MAE 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 MAE-enable bit
> in the th.sxstatus CSR. This CSR is available in HW and can be
> emulated in QEMU.
> 
> This patch also makes the XTheadMae probing mechanism reliable, because
> a test for the right combination of mvendorid, marchid, and mimpid
> is not sufficient to enable MAE.
> 
> Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
> Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
> ---
>   arch/riscv/errata/thead/errata.c | 14 ++++++++++----
>   1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
> index 6e7ee1f16bee..bf6a0a6318ee 100644
> --- a/arch/riscv/errata/thead/errata.c
> +++ b/arch/riscv/errata/thead/errata.c
> @@ -19,6 +19,9 @@
>   #include <asm/patch.h>f
>   #include <asm/vendorid_list.h>
>   
> +#define CSR_TH_SXSTATUS		0x5c0
> +#define SXSTATUS_MAEE		_AC(0x200000, UL)
> +
>   static bool errata_probe_mae(unsigned int stage,
>   			     unsigned long arch_id, unsigned long impid)
>   {
> @@ -28,11 +31,14 @@ static bool errata_probe_mae(unsigned int stage,
>   	if (arch_id != 0 || impid != 0)
>   		return false;
>   

I would raise a little concern about keeping this "if" statement for
arch_id and imp_id after we have probed it in this CSR way. I would like to
remove it and move the CSR probe earlier than RISCV_ALTERNATIVES.

I added CC to guoren for more opinions.

Even T-Head C908 comes in 2023, which supports RVV 1.0 and also keeps MAEE.
But it also supports Svpbmt, and we can perform the switch by clearing the
MAEE bit in CSR_TH_MXSTATUS in M-Mode Software.

Moreover, T-Head MAEE may not be removed in the future of T-Head CPUs. We
can see something from the T-Head C908 User Manual that adds a Security bit
to MAEE. So, it might used in their own TEE implementation and will not be
removed.

However, C908 has arch_id and impid, which are not equal to zero. You can
see it from the C908 boot log [2] from my patch to support K230 [3]. So, if
we have probed MAEE using CSR, you confirmed that this bit will also be set
in the C906 core. We can only probe it this way and no longer use arch_id
and imp_id. And if the arch_id and imp_id probes get removed, we should
also move the csr probe earlier than riscv alternatives.

[1] https://occ-intl-prod.oss-ap-southeast-1.aliyuncs.com/resource//1699268369347/XuanTie-C908-UserManual.pdf
[2] https://gist.github.com/cyyself/b9445f38cc3ba1094924bd41c9086176
[3] https://lore.kernel.org/linux-riscv/tencent_D1180541B4B31C0371DB634A42681A5BF809@qq.com/

Thanks,
Yangyu Chen

> -	if (stage == RISCV_ALTERNATIVES_EARLY_BOOT ||
> -	    stage == RISCV_ALTERNATIVES_MODULE)
> -		return true;
> +	if (stage != RISCV_ALTERNATIVES_EARLY_BOOT &&
> +	    stage != RISCV_ALTERNATIVES_MODULE)
> +		return false;
>   
> -	return false;
> +	if (!(csr_read(CSR_TH_SXSTATUS) & SXSTATUS_MAEE))
> +		return false;
> +
> +	return true;
>   }
>   
>   /*