linux-kernel - Re: [PATCH 06/16] arm64: capabilities: Unify the verification

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Message-ID: <20180126110759.GN5862@e103592.cambridge.arm.com>
Date:   Fri, 26 Jan 2018 11:08:00 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        ckadabi@...eaurora.org, ard.biesheuvel@...aro.org,
        marc.zyngier@....com, catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, jnair@...iumnetworks.com
Subject: Re: [PATCH 06/16] arm64: capabilities: Unify the verification

On Tue, Jan 23, 2018 at 12:27:59PM +0000, Suzuki K Poulose wrote:
> Now that each capability describes how to treat the conflicts
> of CPU cap state vs System wide cap state, we can unify the
> verification logic to a single place.
> 
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
>  arch/arm64/kernel/cpufeature.c | 87 ++++++++++++++++++++++++++----------------
>  1 file changed, 54 insertions(+), 33 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 43c7e992d784..79737034a628 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1228,6 +1228,54 @@ static void __init enable_cpu_capabilities(const struct arm64_cpu_capabilities *
>  }

>  
>  /*
> + * Run through the list of capabilities to check for conflicts.
> + * Returns "false" on conflicts.
> + */
> +static bool __verify_local_cpu_caps(const struct arm64_cpu_capabilities *caps_list)
> +{
> +	bool cpu_has_cap, system_has_cap;
> +	const struct arm64_cpu_capabilities *caps = caps_list;
> +
> +	for (; caps->matches; caps++) {
> +		cpu_has_cap = __this_cpu_has_cap(caps_list, caps->capability);

What's the point of scanning the whole of caps_list?  Don't we already
have the pointer to the right cap struct?

We already know caps->matches is true.  Can't we just call
caps->matches(caps)?  That seemed pretty intuitive to me in the old
code.

> +		system_has_cap =  cpus_have_cap(caps->capability);
> +
> +		if (system_has_cap) {
> +			/*
> +			 * Check if the new CPU misses an advertised feature, which is not
> +			 * safe to miss.
> +			 */
> +			if (!cpu_has_cap && !cpucap_late_cpu_missing_cap_safe(caps))
> +				break;
> +			/*
> +			 * We have to issue enable() irrespective of whether the CPU
> +			 * has it or not, as it is enabeld system wide. It is upto

enabled

> +			 * the call back to take appropriate action on this CPU.
> +			 */
> +			if (caps->enable)
> +				caps->enable(caps);
> +		} else {
> +			/*
> +			 * Check if the CPU has this capability if it isn't safe to
> +			 * have when the system doesn't.
> +			 */

Possibly most of the commenting here is not needed.  The code is pretty
self-explanatory, so the comments may just be adding clutter.

The role of the ->enable() call is the only real subtlety here.

> +			if (cpu_has_cap && !cpucap_late_cpu_have_cap_safe(caps))
> +				break;
> +		}
> +	}
> +
> +	if (caps->matches) {
> +		pr_crit("CPU%d: Detected conflict for capability %d (%s), System: %d, CPU: %d\n",
> +			smp_processor_id(), caps->capability,
> +			caps->desc ? : "no description",

Wouldn't it be a bug for a conflict to occur on a cap with no .desc?

Why can't we just let printk print its default "(null)" for %s
in this case?

Alternatively, is there a reason for any cap not to have a description?

> +			system_has_cap, cpu_has_cap);
> +		return false;
> +	}
> +
> +	return true;
> +}

Perhaps the capability verification procedure could be made a little
clearer by splitting this into two functions:

static bool __verify_local_cpu_cap(const struct arm64_cpu_capabilities *cap)
{
	bool cpu_has_cap = cap->matches(cap, SCOPE_LOCAL_CPU);
	bool system_has_cap =  cpus_have_cap(cap->capability);

	if (system_has_cap) {
		if (!cpu_has_cap && !cpucap_late_cpu_missing_cap_safe(cap))
			goto bad;

		if (cap->enable)
			/* Enable for this cpu if appropriate: */
			cap->enable(cap);
	} else {
		if (cpu_has_cap && !cpucap_late_cpu_have_cap_safe(cap))
			goto bad;
	}

	return true;

bad:
	pr_crit([...]);
	return false;
}

static bool __verify_local_cpu_caps(const struct arm64_cpu_capabilities *caps)
{
	while (caps->matches) {
		if (!__verify_local_cpu_cap(caps))
			return false;

		++caps;
	}

	return true;
}

[...]

Cheers
---Dave