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SubjectRe: [PATCH v2 2/6] arm64: alternative: Apply alternatives early in boot process
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Date
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@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@linaro.org>
> Signed-off-by: Julien Thierry <julien.thierry@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will.deacon@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

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