Re: [PATCH v3 09/21] arm64: cpufeature: Add global feature override facility

[Catalin Marinas <catalin.marinas@arm.com>]

Hi Marc,

On Mon, Jan 11, 2021 at 01:27:59PM +0000, Marc Zyngier wrote:
> Add a facility to globally override a feature, no matter what
> the HW says. Yes, this is dangerous.

Yeah, it's dangerous. We can make it less so if we only allow safe
values (e.g. lower if FTR_UNSIGNED).

> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 9a555809b89c..465d2cb63bfc 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -75,6 +75,8 @@ struct arm64_ftr_reg {
>  	u64				sys_val;
>  	u64				user_val;
>  	const struct arm64_ftr_bits	*ftr_bits;
> +	u64				*override_val;
> +	u64				*override_mask;
>  };

At the arm64_ftr_reg level, we don't have any information about the safe
values for a feature. Could we instead move this to arm64_ftr_bits? We
probably only need a single field. When populating the feature values,
we can make sure it doesn't go above the hardware one.

I attempted a feature modification for MTE here, though I dropped the
entire series in the meantime as we clarified the ARM ARM:

https://lore.kernel.org/linux-arm-kernel/20200515171612.1020-24-catalin.marinas@arm.com/

Srinivas copied it in his patch (but forgot to give credit ;)):

https://lore.kernel.org/linux-arm-msm/1610152163-16554-3-git-send-email-sramana@codeaurora.org/

The above adds a filter function but, instead, just use your mechanism in
this series for idreg.feature setting via cmdline. The arm64_ftr_value()
function extracts the hardware value and lowers it if a cmdline argument
was passed.

-- 
Catalin
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