Re: [PATCH v2 16/20] arm64: Handle shared capability entries

[Dave Martin <Dave.Martin@arm.com>]

On Wed, Jan 31, 2018 at 06:28:03PM +0000, Suzuki K Poulose wrote:
> Some capabilities have different criteria for detection and associated
> actions based on the matching criteria, even though they all share the
> same capability bit. So far we have used multiple entries with the same
> capability bit to handle this. This is prone to errors, as the
> cpu_enable is invoked for each entry, irrespective of whether the
> detection rule applies to the CPU or not. And also this complicates
> other helpers, e.g, __this_cpu_has_cap.
> 
> This patch adds a wrapper entry to cover all the possible variations
> of a capability and ensures :
>  1) The capabilitiy is set when at least one of the entry detects
>  2) Action is only taken for the entries that detects.

I guess this means that where we have a single cpu_enable() method
but complex match criteria that require multiple entries, then that
cpu_enable() method might get called multiple times on a given CPU.

Could be worth a comment if cpu_enable() methods must be robust
against this.

> This avoids explicit checks in the call backs. The only constraint
> here is that, all the entries should have the same "type".
> 
> Cc: Dave Martin <dave.martin@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
>  arch/arm64/include/asm/cpufeature.h |  1 +
>  arch/arm64/kernel/cpu_errata.c      | 53 ++++++++++++++++++++++++++++++++-----
>  arch/arm64/kernel/cpufeature.c      |  7 +++--
>  3 files changed, 50 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 462c35d1a38c..b73247c27f00 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -290,6 +290,7 @@ struct arm64_cpu_capabilities {
>  			bool sign;
>  			unsigned long hwcap;
>  		};
> +		const struct arm64_cpu_capabilities *cap_list;

Should desc, capability, def_scope and/or cpu_enable match for every cap
in such a group?

I'd expected something maybe like this:

struct arm64_cpu_capabilities {
	const char *desc;
	u16 capability;
	struct arm64_capability_match {
		bool (*matches)(const struct arm64_cpu_capabilities *, int);
		int (*cpu_enable)(void);
		union {
			struct { ... midr ... };
			struct { ... sysreg ... };
			const struct arm64_capability_match *list;
		};
>  	};
>  };
>  
> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> index eff5f4e380ac..b4f1c1c1f8ca 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -213,6 +213,36 @@ static void qcom_enable_link_stack_sanitization(
>  	.type = ARM64_CPUCAP_LOCAL_CPU_ERRATUM,			\
>  	CAP_MIDR_RANGE_LIST(midr_list)
>  
> +/*
> + * Generic helper for handling capabilties with multiple (match,enable) pairs
> + * of call backs, sharing the same capability bit.
> + * Iterate over each entry to see if at least one matches.
> + */
> +static bool multi_entry_cap_matches(const struct arm64_cpu_capabilities *entry,
> +				    int scope)
> +{
> +	const struct arm64_cpu_capabilities *caps = entry->cap_list;
> +
> +	for (; caps->matches; caps++)
> +		if (caps->matches(caps, scope))
> +			return true;
> +	return false;
> +}
> +
> +/*
> + * Take appropriate action for all matching entries in the shared capability
> + * entry.
> + */
> +static void multi_entry_cap_cpu_enable(const struct arm64_cpu_capabilities *entry)
> +{
> +	const struct arm64_cpu_capabilities *caps = entry->cap_list;
> +
> +	for (; caps->matches; caps++)
> +		if (caps->matches(caps, SCOPE_LOCAL_CPU) &&
> +		    caps->cpu_enable)
> +			caps->cpu_enable(caps);
> +}
> +
>  #ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
>  
>  /*
> @@ -229,6 +259,18 @@ static const struct midr_range arm64_bp_harden_psci_cpus[] = {
>  	{},
>  };
>  
> +static const struct arm64_cpu_capabilities arm64_bp_harden_list[] = {
> +	{
> +		CAP_MIDR_RANGE_LIST(arm64_bp_harden_psci_cpus),
> +		.cpu_enable = enable_psci_bp_hardening,
> +	},
> +	{
> +		CAP_MIDR_ALL_VERSIONS(MIDR_QCOM_FALKOR_V1),
> +		.cpu_enable = qcom_enable_link_stack_sanitization,
> +	},
> +	{},
> +};
> +
>  #endif
>  
>  const struct arm64_cpu_capabilities arm64_errata[] = {
> @@ -365,13 +407,10 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
>  #ifdef CONFIG_HARDEN_BRANCH_PREDICTOR
>  	{
>  		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
> -		ERRATA_MIDR_RANGE_LIST(arm64_bp_harden_psci_cpus),
> -		.cpu_enable = enable_psci_bp_hardening,
> -	},
> -	{
> -		.capability = ARM64_HARDEN_BRANCH_PREDICTOR,
> -		ERRATA_MIDR_ALL_VERSIONS(MIDR_QCOM_FALKOR_V1),
> -		.cpu_enable = qcom_enable_link_stack_sanitization,
> +		.type = ARM64_CPUCAP_LOCAL_CPU_ERRATUM,
> +		.matches = multi_entry_cap_matches,
> +		.cpu_enable = multi_entry_cap_cpu_enable,
> +		.cap_list = arm64_bp_harden_list,
>  	},
>  	{
>  		.capability = ARM64_HARDEN_BP_POST_GUEST_EXIT,
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 65a8e5cc600c..13e30c1b1e99 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1181,9 +1181,8 @@ static bool __this_cpu_has_cap(const struct arm64_cpu_capabilities *cap_array,
>  		return false;
>  
>  	for (caps = cap_array; caps->matches; caps++)
> -		if (caps->capability == cap &&
> -		    caps->matches(caps, SCOPE_LOCAL_CPU))
> -			return true;
> +		if (caps->capability == cap)
> +			return caps->matches(caps, SCOPE_LOCAL_CPU);

If we went for my capability { cap; match criteria or list; } approach,
would it still be necessary to iterate over the whole list here?

This seems preferable if this function is used by other paths that
don't expect it to be so costly.  Currently I only see a call in
arch/arm64/kvm/handle_exit.c:handle_exit_early() for the SError case --
which is probably not expected to be a fast path.

[...]

Cheers
---Dave