linux-kernel - Re: [PATCH v5 02/27] arm64: cpufeature: Use alternatives for VHE cpu

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Date:   Wed, 12 Sep 2018 11:28:20 +0100
From:   James Morse <james.morse@....com>
To:     Julien Thierry <julien.thierry@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, daniel.thompson@...aro.org,
        joel@...lfernandes.org, marc.zyngier@....com, mark.rutland@....com,
        christoffer.dall@....com, catalin.marinas@....com,
        will.deacon@....com, Suzuki K Poulose <suzuki.poulose@....com>
Subject: Re: [PATCH v5 02/27] arm64: cpufeature: Use alternatives for VHE
 cpu_enable

Hi Julien,

On 28/08/18 16:51, Julien Thierry wrote:
> The cpu_enable callback for VHE feature requires all alternatives to have
> been applied. This prevents applying VHE alternative separately from the
> rest.
> 
> Use an alternative depending on VHE feature to know whether VHE
> alternatives have already been applied.

> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 1e433ac..3bc1c8b 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1022,8 +1024,15 @@ static void cpu_copy_el2regs(const struct arm64_cpu_capabilities *__unused)
>  	 * that, freshly-onlined CPUs will set tpidr_el2, so we don't need to
>  	 * do anything here.
>  	 */
> -	if (!alternatives_applied)
> -		write_sysreg(read_sysreg(tpidr_el1), tpidr_el2);
> +	asm volatile(ALTERNATIVE(
> +		"mrs	%0, tpidr_el1\n"
> +		"msr	tpidr_el2, %0",
> +		"nop\n"
> +		"nop",
> +		ARM64_HAS_VIRT_HOST_EXTN)
> +		: "+r" (tmp)
> +		:
> +		: "memory");
>  }
>  #endif

Catalin's preference was to keep this all in C:
https://patchwork.kernel.org/patch/10007977/
, for which we need to know if 'the' alternative has been applied.

I suspect there may be more registers in this list if we have to switch to
another EL2 register using alternatives. (but I don't have an example).

Could we make 'alternatives_applied' a macro that takes the cap as an argument?


Thanks,

James