[PATCH v2 2/6] arm64: alternative: Apply alternatives early in boot process

[Julien Thierry]

Hi,

In order to prepare the v3 of this patchset, I'd like people's opinion 
on what this patch does. More below.

On 17/01/18 11:54, Julien Thierry wrote:
> From: Daniel Thompson <daniel.thompson at linaro.org>
> 
> Currently alternatives are applied very late in the boot process (and
> a long time after we enable scheduling). Some alternative sequences,
> such as those that alter the way CPU context is stored, must be applied
> much earlier in the boot sequence.
> 
> Introduce apply_alternatives_early() to allow some alternatives to be
> applied immediately after we detect the CPU features of the boot CPU.
> 
> Signed-off-by: Daniel Thompson <daniel.thompson at linaro.org>
> Signed-off-by: Julien Thierry <julien.thierry at arm.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will.deacon at arm.com>
> ---
>   arch/arm64/include/asm/alternative.h |  1 +
>   arch/arm64/kernel/alternative.c      | 39 +++++++++++++++++++++++++++++++++---
>   arch/arm64/kernel/smp.c              |  6 ++++++
>   3 files changed, 43 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
> index 4a85c69..1fc1cdb 100644
> --- a/arch/arm64/include/asm/alternative.h
> +++ b/arch/arm64/include/asm/alternative.h
> @@ -20,6 +20,7 @@ struct alt_instr {
>   	u8  alt_len;		/* size of new instruction(s), <= orig_len */
>   };
> 
> +void __init apply_alternatives_early(void);
>   void __init apply_alternatives_all(void);
>   void apply_alternatives(void *start, size_t length);
> 
> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
> index 6dd0a3a3..78051d4 100644
> --- a/arch/arm64/kernel/alternative.c
> +++ b/arch/arm64/kernel/alternative.c
> @@ -28,6 +28,18 @@
>   #include <asm/sections.h>
>   #include <linux/stop_machine.h>
> 
> +/*
> + * early-apply features are detected using only the boot CPU and checked on
> + * secondary CPUs startup, even then,
> + * These early-apply features should only include features where we must
> + * patch the kernel very early in the boot process.
> + *
> + * Note that the cpufeature logic *must* be made aware of early-apply
> + * features to ensure they are reported as enabled without waiting
> + * for other CPUs to boot.
> + */
> +#define EARLY_APPLY_FEATURE_MASK BIT(ARM64_HAS_SYSREG_GIC_CPUIF)
> +

Following the change in the cpufeature infrastructure, 
ARM64_HAS_SYSREG_GIC_CPUIF will have the scope 
ARM64_CPUCAP_SCOPE_BOOT_CPU in order to be checked early in the boot 
process.

Now, regarding the early application of alternative, I am wondering 
whether we can apply all the alternatives associated with SCOPE_BOOT 
features that *do not* have a cpu_enable callback.

Otherwise we can keep the macro to list individually each feature that 
is patchable at boot time as the current patch does (or put this info in 
a flag within the arm64_cpu_capabilities structure).

Any thoughts or preferences on this?

Thanks,

>   #define __ALT_PTR(a,f)		((void *)&(a)->f + (a)->f)
>   #define ALT_ORIG_PTR(a)		__ALT_PTR(a, orig_offset)
>   #define ALT_REPL_PTR(a)		__ALT_PTR(a, alt_offset)
> @@ -105,7 +117,8 @@ static u32 get_alt_insn(struct alt_instr *alt, __le32 *insnptr, __le32 *altinsnp
>   	return insn;
>   }
> 
> -static void __apply_alternatives(void *alt_region, bool use_linear_alias)
> +static void __apply_alternatives(void *alt_region,  bool use_linear_alias,
> +				 unsigned long feature_mask)
>   {
>   	struct alt_instr *alt;
>   	struct alt_region *region = alt_region;
> @@ -115,6 +128,9 @@ static void __apply_alternatives(void *alt_region, bool use_linear_alias)
>   		u32 insn;
>   		int i, nr_inst;
> 
> +		if ((BIT(alt->cpufeature) & feature_mask) == 0)
> +			continue;
> +
>   		if (!cpus_have_cap(alt->cpufeature))
>   			continue;
> 
> @@ -138,6 +154,21 @@ static void __apply_alternatives(void *alt_region, bool use_linear_alias)
>   }
> 
>   /*
> + * This is called very early in the boot process (directly after we run
> + * a feature detect on the boot CPU). No need to worry about other CPUs
> + * here.
> + */
> +void apply_alternatives_early(void)
> +{
> +	struct alt_region region = {
> +		.begin	= (struct alt_instr *)__alt_instructions,
> +		.end	= (struct alt_instr *)__alt_instructions_end,
> +	};
> +
> +	__apply_alternatives(&region, true, EARLY_APPLY_FEATURE_MASK);
> +}
> +
> +/*
>    * We might be patching the stop_machine state machine, so implement a
>    * really simple polling protocol here.
>    */
> @@ -156,7 +187,9 @@ static int __apply_alternatives_multi_stop(void *unused)
>   		isb();
>   	} else {
>   		BUG_ON(patched);
> -		__apply_alternatives(&region, true);
> +
> +		__apply_alternatives(&region, true, ~EARLY_APPLY_FEATURE_MASK);
> +
>   		/* Barriers provided by the cache flushing */
>   		WRITE_ONCE(patched, 1);
>   	}
> @@ -177,5 +210,5 @@ void apply_alternatives(void *start, size_t length)
>   		.end	= start + length,
>   	};
> 
> -	__apply_alternatives(&region, false);
> +	__apply_alternatives(&region, false, -1);
>   }
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 551eb07..37361b5 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -453,6 +453,12 @@ void __init smp_prepare_boot_cpu(void)
>   	 * cpuinfo_store_boot_cpu() above.
>   	 */
>   	update_cpu_errata_workarounds();
> +	/*
> +	 * We now know enough about the boot CPU to apply the
> +	 * alternatives that cannot wait until interrupt handling
> +	 * and/or scheduling is enabled.
> +	 */
> +	apply_alternatives_early();
>   }
> 
>   static u64 __init of_get_cpu_mpidr(struct device_node *dn)
> --
> 1.9.1
> 

-- 
Julien Thierry