Re: [PATCH 15/16] arm64: Delay enabling hardware DBM feature

[Dave Martin]

On Tue, Jan 23, 2018 at 12:28:08PM +0000, Suzuki K Poulose wrote:
> We enable hardware DBM bit in a capable CPU, very early in the
> boot via __cpu_setup. This doesn't give us a flexibility of
> optionally disable the feature, as the clearing the bit
> is a bit costly as the TLB can cache the settings. Instead,
> we delay enabling the feature until the CPU is brought up
> into the kernel. We use the feature capability mechanism
> to handle it.
> 
> The hardware DBM is a non-conflicting feature. i.e, the kernel
> can safely run with a mix of CPUs with some using the feature
> and the others don't. So, it is safe for a late CPU to have
> this capability and enable it, even if the active CPUs don't.
> 
> To get this handled properly by the infrastructure, we
> unconditionally set the capability and only enable it
> on CPUs which really have the feature. Adds a new type
> of feature to the capability infrastructure which
> ignores the conflict in a late CPU.
> 
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> ---
>  arch/arm64/include/asm/cpucaps.h    |  3 ++-
>  arch/arm64/include/asm/cpufeature.h |  8 +++++++
>  arch/arm64/kernel/cpufeature.c      | 42 +++++++++++++++++++++++++++++++++++++
>  arch/arm64/mm/proc.S                |  5 +----
>  4 files changed, 53 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
> index bb263820de13..8df80cc828ac 100644
> --- a/arch/arm64/include/asm/cpucaps.h
> +++ b/arch/arm64/include/asm/cpucaps.h
> @@ -45,7 +45,8 @@
>  #define ARM64_HARDEN_BRANCH_PREDICTOR		24
>  #define ARM64_HARDEN_BP_POST_GUEST_EXIT		25
>  #define ARM64_HAS_RAS_EXTN			26
> +#define ARM64_HW_DBM				27
>  
> -#define ARM64_NCAPS				27
> +#define ARM64_NCAPS				28
>  
>  #endif /* __ASM_CPUCAPS_H */
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 70712de687c7..243ec7c77c79 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -126,6 +126,14 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
>   */
>  #define ARM64_CPUCAP_STRICT_CPU_LOCAL_FEATURE	\
>  	(ARM64_CPUCAP_SCOPE_LOCAL_CPU | ARM64_CPUCAP_LATE_CPU_SAFE_TO_MISS)
> +/*
> + * CPU feature detected on each local CPU. It is safe for a late CPU to
> + * either have it or not.
> + */
> +#define ARM64_CPUCAP_WEAK_CPU_LOCAL_FEATURE	 \
> +	(ARM64_CPUCAP_SCOPE_LOCAL_CPU		|\
> +	 ARM64_CPUCAP_LATE_CPU_SAFE_TO_MISS	|\
> +	 ARM64_CPUCAP_LATE_CPU_SAFE_TO_HAVE)

OK, so this is similar to my suggestion for HAS_NO_HW_PREFETCH (though
that need not have the same answer -- I was speculating there).

Nit: tab between | and \?

>  struct arm64_cpu_capabilities {
>  	const char *desc;
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 2627a836e99d..8af755b8219d 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -894,6 +894,35 @@ static int __init parse_kpti(char *str)
>  __setup("kpti=", parse_kpti);
>  #endif	/* CONFIG_UNMAP_KERNEL_AT_EL0 */
>  
> +#ifdef CONFIG_ARM64_HW_AFDBM
> +static bool has_hw_dbm(const struct arm64_cpu_capabilities *entry, int scope)
> +{
> +	/*
> +	 * DBM is a non-conflicting feature. i.e, the kernel can safely run
> +	 * a mix of CPUs with and without the feature. So, we unconditionally
> +	 * enable the capability to allow any late CPU to use the feature.
> +	 * We only enable the control bits on the CPU, if it actually supports.
> +	 */
> +	return true;
> +}
> +
> +static inline void __cpu_enable_hw_dbm(void)
> +{
> +	u64 tcr = read_sysreg(tcr_el1) | TCR_HD;
> +
> +	write_sysreg(tcr, tcr_el1);
> +	isb();

Do we need this isb?  Do we care exactly when setting TCR_HD appears
to take effect?

> +}
> +
> +static int cpu_enable_hw_dbm(struct arm64_cpu_capabilities const *cap)
> +{
> +	if (has_cpuid_feature(cap, SCOPE_LOCAL_CPU))
> +		__cpu_enable_hw_dbm();
> +
> +	return 0;
> +}
> +#endif
> +
>  static int cpu_copy_el2regs(const struct arm64_cpu_capabilities *__unused)
>  {
>  	/*
> @@ -1052,6 +1081,19 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.enable = cpu_clear_disr,
>  	},
>  #endif /* CONFIG_ARM64_RAS_EXTN */
> +#ifdef CONFIG_ARM64_HW_AFDBM
> +	{
> +		.desc = "Hardware pagetable Dirty Bit Management",
> +		.type = ARM64_CPUCAP_WEAK_CPU_LOCAL_FEATURE,
> +		.capability = ARM64_HW_DBM,
> +		.sys_reg = SYS_ID_AA64MMFR1_EL1,
> +		.sign = FTR_UNSIGNED,
> +		.field_pos = ID_AA64MMFR1_HADBS_SHIFT,
> +		.min_field_value = 2,
> +		.matches = has_hw_dbm,

Can't we use has_cpuid_feature here?  Why do we need a fake .matches and
then code the check manually in the enable mathod?

I may be missing something here.

[...]

Cheers
---Dave