Message-ID: <20180207103754.GU5862@e103592.cambridge.arm.com>
Date:   Wed, 7 Feb 2018 10:37:55 +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,
        dave.martin@....com
Subject: Re: [PATCH v2 04/20] arm64: capabilities: Prepare for fine grained
 capabilities

On Wed, Jan 31, 2018 at 06:27:51PM +0000, Suzuki K Poulose wrote:
> We use arm64_cpu_capabilities to represent CPU ELF HWCAPs exposed
> to the userspace and the CPU hwcaps used by the kernel, which
> include cpu features and CPU errata work arounds. Capabilities
> have some properties that decide how they should be treated :
> 
>  1) Detection, i.e scope : A cap could be "detected" either :
>     - if it is present on at least one CPU (SCOPE_LOCAL_CPU)
> 	Or
>     - if it is present on all the CPUs (SCOPE_SYSTEM)
> 
>  2) When is it enabled ? - A cap is treated as "enabled" when the
>   system takes some action based on whether the capability is detected or
>   not. e.g, setting some control register, patching the kernel code.
>   Right now, we treat all caps are enabled at boot-time, after all
>   the CPUs are brought up by the kernel. But there are certain caps,
>   which are enabled early during the boot (e.g, VHE, GIC_CPUIF for NMI)
>   and kernel starts using them, even before the secondary CPUs are brought
>   up. We would need a way to describe this for each capability.
> 
>  3) Conflict on a late CPU - When a CPU is brought up, it is checked
>   against the caps that are known to be enabled on the system (via
>   verify_local_cpu_capabilities()). Based on the state of the capability
>   on the CPU vs. that of System we could have the following combinations
>   of conflict.
> 
> 	x-----------------------------x
> 	| Type	| System   | Late CPU |
> 	------------------------------|
> 	|  a    |   y      |    n     |
> 	------------------------------|
> 	|  b    |   n      |    y     |
> 	x-----------------------------x
> 
>   Case (a) is not permitted for caps which are system features, which the
>   system expects all the CPUs to have (e.g VHE). While (a) is ignored for
>   all errata work arounds. However, there could be exceptions to the plain
>   filtering approach. e.g, KPTI is an optional feature for a late CPU as
>   long as the system already enables it.
> 
>   Case (b) is not permitted for errata work arounds which requires some
>   work around, which cannot be delayed. And we ignore (b) for features. Here,
>   yet again, KPTI is an exception, where if a late CPU needs KPTI we are too
>   late to enable it (because we change the allocation of ASIDs etc).
> 
> So this calls for a lot more fine grained behavior for each capability.
> And if we define all the attributes to control their behavior properly,
> we may be able to use a single table for the CPU hwcaps (which cover
> errata and features, not the ELF HWCAPs). This is a prepartory step
> to get there. More bits would be added for the properties listed above.
> 
> We are going to use a bit-mask to encode all the properties of a capabilities.
> This patch encodes the "SCOPE" of the capability.
> 
> As such there is no change in how the capabilities are treated.
> 
> Cc: Dave Martin <dave.martin@....com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>

A few minor nits in the documentation, otherwise

Reviewed-by: Dave Martin <Dave.Martin@....com>

> ---
>  arch/arm64/include/asm/cpufeature.h | 90 ++++++++++++++++++++++++++++++++++---
>  arch/arm64/kernel/cpu_errata.c      |  8 ++--
>  arch/arm64/kernel/cpufeature.c      | 38 ++++++++--------
>  3 files changed, 107 insertions(+), 29 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 7925e40c6ded..05da54f1b4c7 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -86,16 +86,89 @@ struct arm64_ftr_reg {
>  
>  extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
>  
> -/* scope of capability check */
> -enum {
> -	SCOPE_SYSTEM,
> -	SCOPE_LOCAL_CPU,
> -};
> +/*
> + * CPU capabilities:
> + *
> + * We use arm64_cpu_capabilities to represent system features, errata work
> + * arounds (both used internally by kernel and tracked in cpu_hwcaps) and
> + * ELF HWCAPs (which are exposed to user).
> + *
> + * To support systems with heterogeneous CPUs, we need to make sure that we
> + * detect the capabilities correctly on the system and take appropriate
> + * measures to ensure there are not incompatibilities.
> + *
> + * This comment tries to explain how we treat the capabilities.
> + * Each capability has the following list of attributes :
> + *
> + * 1) Scope of Detection : The system detects a given capability by performing
> + *    some checks at runtime. This could be, e.g, checking the value of a field
> + *    in CPU ID feature register or checking the cpu model. The capability
> + *    provides a call back ( @matches() ) to perform the check.
> + *    Scope defines how the checks should be performed. There are two cases:
> + *
> + *     a) SCOPE_LOCAL_CPU: check all the CPUs and "detect" if at least one
> + *        matches. This implies, we have to run the check on all the booting
> + *        CPUs, until the system decides that state of the capability is finalised.
> + *        (See section 2 below)
> + *		Or
> + *     b) SCOPE_SYSTEM: check all the CPUs and "detect" if all the CPUs matches.
> + *        This implies, we run the check only once, when the system decides to
> + *        finalise the state of the capability. If the capability relies on a
> + *        field in one of the CPU ID feature registers, we use the sanitised
> + *        value of the register from the CPU feature infrastructure to make
> + *        the decision.
> + *    The process of detection is usually denoted by "update" capability state
> + *    in the code.
> + *
> + * 2) Finalise the state : The kernel should finalise the state of a capability
> + *    at some point during its execution and take necessary actions if any. Usually,
> + *    this is done, after all the boot-time enabled CPUs are brought up by the
> + *    kernel, so that it can make better decision based on the available set
> + *    of CPUs. However, there are some special cases, where the action is taken
> + *    during the early boot by the primary boot CPU. (e.g, running the kernel at
> + *    EL2 with Virtualisation Host Extensions). The kernel usually disallows
> + *    any changes to the state of a capability once it finalises the capability
> + *    and takes any action, as it may be impossible to execute the actions safely.
> + *
> + * 3) Verification: When a CPU is brought online (e.g, by user or by the kernel),
> + *    the kernel should make sure that it is safe to use the CPU, by verifying
> + *    that the CPU is compliant with the state of the capabilities established

Nit: can we say "finalised" instead of "established"?

There could be doubt about precisely what "established" means.
"Finalised" is clearly defined in (2) -- I'm assuming that's the
intended meaning here (?)

> + *    already. This happens via :
> + *    secondary_start_kernel()-> check_local_cpu_capabilities()	->
> + *      check_early_cpu_features() && verify_local_cpu_capabilities()

Nit: Maybe just say "via secondart_start_kernel()"?  Too much detail
about the exact code flow may become wrong in the future when someone
refactors the code.

> + *
> + *    As explained in (2) above, capabilities could be finalised at different
> + *    points in the execution. Each CPU is verified against the "finalised"
> + *    capabilities and if there is a conflict, the kernel takes an action, based
> + *    on the severity (e.g, a CPU could be prevented from booting or cause a
> + *    kernel panic). The CPU is allowed to "affect" the state of the capability,
> + *    if it has not been finalised already.
> + *
> + * 4) Action: As mentioned in (2), the kernel can take an action for each detected
> + *    capability, on all CPUs on the system. This is always initiated only after

Nit: maybe clarify what an action is, e.g.
"Appropriate actions include patching in alternatives, turning on an
architectural feature or activating errata workarounds."

Can we can that it is the job of the cpu_enable() method to perform the
appropriate action, or is that not universally true?

> + *    the capability is finalised. This is usally denoted by "enabling" the
> + *    capability. The actions are initiated as follows :
> + *	a) Action is triggered on all online CPUs, after the capability is
> + *	finalised, invoked within the stop_machine() context from
> + *	enable_cpu_capabilitie().
> + *
> + *	b) Any late CPU, brought up after (1), the action is triggered via:
> + *		check_local_cpu_capabilities() -> verify_local_cpu_capabilities()
> + *
> + */
> +
> +
> +/* Decide how the capability is detected. On a local CPU vs System wide */
> +#define ARM64_CPUCAP_SCOPE_MASK			0x3
> +#define ARM64_CPUCAP_SCOPE_LOCAL_CPU		((u16)BIT(0))
> +#define ARM64_CPUCAP_SCOPE_SYSTEM		((u16)BIT(1))
> +#define SCOPE_SYSTEM				ARM64_CPUCAP_SCOPE_SYSTEM
> +#define SCOPE_LOCAL_CPU				ARM64_CPUCAP_SCOPE_LOCAL_CPU
>  
>  struct arm64_cpu_capabilities {
>  	const char *desc;
>  	u16 capability;
> -	int def_scope;			/* default scope */
> +	u16 type;
>  	bool (*matches)(const struct arm64_cpu_capabilities *caps, int scope);
>  	/*
>  	 * Take the appropriate actions to enable this capability for this CPU.
> @@ -120,6 +193,11 @@ struct arm64_cpu_capabilities {
>  	};
>  };
>  
> +static inline int cpucap_default_scope(const struct arm64_cpu_capabilities *cap)
> +{
> +	return cap->type & ARM64_CPUCAP_SCOPE_MASK;
> +}
> +
>  extern DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
>  extern struct static_key_false cpu_hwcap_keys[ARM64_NCAPS];
>  extern struct static_key_false arm64_const_caps_ready;
> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> index 9de0ee5d73c0..328c5a031e45 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -175,14 +175,14 @@ static void qcom_enable_link_stack_sanitization(
>  #endif	/* CONFIG_HARDEN_BRANCH_PREDICTOR */
>  
>  #define MIDR_RANGE(model, min, max) \
> -	.def_scope = SCOPE_LOCAL_CPU, \
> +	.type = ARM64_CPUCAP_SCOPE_LOCAL_CPU, \
>  	.matches = is_affected_midr_range, \
>  	.midr_model = model, \
>  	.midr_range_min = min, \
>  	.midr_range_max = max
>  
>  #define MIDR_ALL_VERSIONS(model) \
> -	.def_scope = SCOPE_LOCAL_CPU, \
> +	.type = ARM64_CPUCAP_SCOPE_LOCAL_CPU, \
>  	.matches = is_affected_midr_range, \
>  	.midr_model = model, \
>  	.midr_range_min = 0, \
> @@ -286,7 +286,7 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
>  		.desc = "Mismatched cache line size",
>  		.capability = ARM64_MISMATCHED_CACHE_LINE_SIZE,
>  		.matches = has_mismatched_cache_line_size,
> -		.def_scope = SCOPE_LOCAL_CPU,
> +		.type = ARM64_CPUCAP_SCOPE_LOCAL_CPU,
>  		.cpu_enable = cpu_enable_trap_ctr_access,
>  	},
>  #ifdef CONFIG_QCOM_FALKOR_ERRATUM_1003
> @@ -300,7 +300,7 @@ const struct arm64_cpu_capabilities arm64_errata[] = {
>  	{
>  		.desc = "Qualcomm Technologies Kryo erratum 1003",
>  		.capability = ARM64_WORKAROUND_QCOM_FALKOR_E1003,
> -		.def_scope = SCOPE_LOCAL_CPU,
> +		.type = ARM64_CPUCAP_SCOPE_LOCAL_CPU,
>  		.midr_model = MIDR_QCOM_KRYO,
>  		.matches = is_kryo_midr,
>  	},
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index f0cd165c75e9..8d22f0ef0927 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -921,7 +921,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		.desc = "GIC system register CPU interface",
>  		.capability = ARM64_HAS_SYSREG_GIC_CPUIF,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = has_useable_gicv3_cpuif,
>  		.sys_reg = SYS_ID_AA64PFR0_EL1,
>  		.field_pos = ID_AA64PFR0_GIC_SHIFT,
> @@ -932,7 +932,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		.desc = "Privileged Access Never",
>  		.capability = ARM64_HAS_PAN,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = has_cpuid_feature,
>  		.sys_reg = SYS_ID_AA64MMFR1_EL1,
>  		.field_pos = ID_AA64MMFR1_PAN_SHIFT,
> @@ -945,7 +945,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		.desc = "LSE atomic instructions",
>  		.capability = ARM64_HAS_LSE_ATOMICS,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = has_cpuid_feature,
>  		.sys_reg = SYS_ID_AA64ISAR0_EL1,
>  		.field_pos = ID_AA64ISAR0_ATOMICS_SHIFT,
> @@ -956,14 +956,14 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		.desc = "Software prefetching using PRFM",
>  		.capability = ARM64_HAS_NO_HW_PREFETCH,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = has_no_hw_prefetch,
>  	},
>  #ifdef CONFIG_ARM64_UAO
>  	{
>  		.desc = "User Access Override",
>  		.capability = ARM64_HAS_UAO,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = has_cpuid_feature,
>  		.sys_reg = SYS_ID_AA64MMFR2_EL1,
>  		.field_pos = ID_AA64MMFR2_UAO_SHIFT,
> @@ -977,21 +977,21 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  #ifdef CONFIG_ARM64_PAN
>  	{
>  		.capability = ARM64_ALT_PAN_NOT_UAO,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = cpufeature_pan_not_uao,
>  	},
>  #endif /* CONFIG_ARM64_PAN */
>  	{
>  		.desc = "Virtualization Host Extensions",
>  		.capability = ARM64_HAS_VIRT_HOST_EXTN,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = runs_at_el2,
>  		.cpu_enable = cpu_copy_el2regs,
>  	},
>  	{
>  		.desc = "32-bit EL0 Support",
>  		.capability = ARM64_HAS_32BIT_EL0,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = has_cpuid_feature,
>  		.sys_reg = SYS_ID_AA64PFR0_EL1,
>  		.sign = FTR_UNSIGNED,
> @@ -1001,21 +1001,21 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		.desc = "Reduced HYP mapping offset",
>  		.capability = ARM64_HYP_OFFSET_LOW,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = hyp_offset_low,
>  	},
>  #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
>  	{
>  		.desc = "Kernel page table isolation (KPTI)",
>  		.capability = ARM64_UNMAP_KERNEL_AT_EL0,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = unmap_kernel_at_el0,
>  	},
>  #endif
>  	{
>  		/* FP/SIMD is not implemented */
>  		.capability = ARM64_HAS_NO_FPSIMD,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.min_field_value = 0,
>  		.matches = has_no_fpsimd,
>  	},
> @@ -1023,7 +1023,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		.desc = "Data cache clean to Point of Persistence",
>  		.capability = ARM64_HAS_DCPOP,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = has_cpuid_feature,
>  		.sys_reg = SYS_ID_AA64ISAR1_EL1,
>  		.field_pos = ID_AA64ISAR1_DPB_SHIFT,
> @@ -1033,8 +1033,8 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  #ifdef CONFIG_ARM64_SVE
>  	{
>  		.desc = "Scalable Vector Extension",
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.capability = ARM64_SVE,
> -		.def_scope = SCOPE_SYSTEM,
>  		.sys_reg = SYS_ID_AA64PFR0_EL1,
>  		.sign = FTR_UNSIGNED,
>  		.field_pos = ID_AA64PFR0_SVE_SHIFT,
> @@ -1047,7 +1047,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		.desc = "RAS Extension Support",
>  		.capability = ARM64_HAS_RAS_EXTN,
> -		.def_scope = SCOPE_SYSTEM,
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,
>  		.matches = has_cpuid_feature,
>  		.sys_reg = SYS_ID_AA64PFR0_EL1,
>  		.sign = FTR_UNSIGNED,
> @@ -1059,16 +1059,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{},
>  };
>  
> -#define HWCAP_CAP(reg, field, s, min_value, type, cap)	\
> +#define HWCAP_CAP(reg, field, s, min_value, cap_type, cap)	\
>  	{							\
>  		.desc = #cap,					\
> -		.def_scope = SCOPE_SYSTEM,			\
> +		.type = ARM64_CPUCAP_SCOPE_SYSTEM,		\
>  		.matches = has_cpuid_feature,			\
>  		.sys_reg = reg,					\
>  		.field_pos = field,				\
>  		.sign = s,					\
>  		.min_field_value = min_value,			\
> -		.hwcap_type = type,				\
> +		.hwcap_type = cap_type,				\
>  		.hwcap = cap,					\
>  	}
>  
> @@ -1161,7 +1161,7 @@ static void __init setup_elf_hwcaps(const struct arm64_cpu_capabilities *hwcaps)
>  	/* We support emulation of accesses to CPU ID feature registers */
>  	elf_hwcap |= HWCAP_CPUID;
>  	for (; hwcaps->matches; hwcaps++)
> -		if (hwcaps->matches(hwcaps, hwcaps->def_scope))
> +		if (hwcaps->matches(hwcaps, cpucap_default_scope(hwcaps)))
>  			cap_set_elf_hwcap(hwcaps);
>  }
>  
> @@ -1188,7 +1188,7 @@ static void update_cpu_capabilities(const struct arm64_cpu_capabilities *caps,
>  			    const char *info)
>  {
>  	for (; caps->matches; caps++) {
> -		if (!caps->matches(caps, caps->def_scope))
> +		if (!caps->matches(caps, cpucap_default_scope(caps)))
>  			continue;
>  
>  		if (!cpus_have_cap(caps->capability) && caps->desc)
> -- 
> 2.14.3
> 
> 
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