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    SubjectRe: [PATCH v8 15/26] arm64: alternative: Apply alternatives early in boot process
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    On 08/01/2019 15:20, Julien Thierry wrote:
    > Hi Suzuki,
    >
    > On 08/01/2019 14:51, Suzuki K Poulose wrote:
    >> Hi Julien,
    >>
    >> On 08/01/2019 14:07, 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_boot_alternatives() 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>
    >>> [julien.thierry@arm.com: rename to fit new cpufeature framework better,
    >>>              apply BOOT_SCOPE feature early in boot]
    >>> Signed-off-by: Julien Thierry <julien.thierry@arm.com>
    >>> Cc: Catalin Marinas <catalin.marinas@arm.com>
    >>> Cc: Will Deacon <will.deacon@arm.com>
    >>> Cc: Christoffer Dall <christoffer.dall@arm.com>
    >>> Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
    >>> ---
    >>>   arch/arm64/include/asm/alternative.h |  1 +
    >>>   arch/arm64/include/asm/cpufeature.h  |  4 ++++
    >>>   arch/arm64/kernel/alternative.c      | 43
    >>> +++++++++++++++++++++++++++++++-----
    >>>   arch/arm64/kernel/cpufeature.c       |  6 +++++
    >>>   arch/arm64/kernel/smp.c              |  7 ++++++
    >>>   5 files changed, 56 insertions(+), 5 deletions(-)
    >>>
    >>> diff --git a/arch/arm64/include/asm/alternative.h
    >>> b/arch/arm64/include/asm/alternative.h
    >>> index 9806a23..b9f8d78 100644
    >>> --- a/arch/arm64/include/asm/alternative.h
    >>> +++ b/arch/arm64/include/asm/alternative.h
    >>> @@ -25,6 +25,7 @@ struct alt_instr {
    >>>   typedef void (*alternative_cb_t)(struct alt_instr *alt,
    >>>                    __le32 *origptr, __le32 *updptr, int nr_inst);
    >>>   +void __init apply_boot_alternatives(void);
    >>>   void __init apply_alternatives_all(void);
    >>>   bool alternative_is_applied(u16 cpufeature);
    >>>   diff --git a/arch/arm64/include/asm/cpufeature.h
    >>> b/arch/arm64/include/asm/cpufeature.h
    >>> index 89c3f31..e505e1f 100644
    >>> --- a/arch/arm64/include/asm/cpufeature.h
    >>> +++ b/arch/arm64/include/asm/cpufeature.h
    >>> @@ -391,6 +391,10 @@ static inline int cpucap_default_scope(const
    >>> struct arm64_cpu_capabilities *cap)
    >>>   extern struct static_key_false cpu_hwcap_keys[ARM64_NCAPS];
    >>>   extern struct static_key_false arm64_const_caps_ready;
    >>>   +/* ARM64 CAPS + alternative_cb */
    >>> +#define ARM64_NPATCHABLE (ARM64_NCAPS + 1)
    >>> +extern DECLARE_BITMAP(boot_capabilities, ARM64_NPATCHABLE);
    >>> +
    >>>   #define for_each_available_cap(cap)        \
    >>>       for_each_set_bit(cap, cpu_hwcaps, ARM64_NCAPS)
    >>>   diff --git a/arch/arm64/kernel/alternative.c
    >>> b/arch/arm64/kernel/alternative.c
    >>> index c947d22..a9b4677 100644
    >>> --- a/arch/arm64/kernel/alternative.c
    >>> +++ b/arch/arm64/kernel/alternative.c
    >>> @@ -155,7 +155,8 @@ static void clean_dcache_range_nopatch(u64 start,
    >>> u64 end)
    >>>       } while (cur += d_size, cur < end);
    >>>   }
    >>>   -static void __apply_alternatives(void *alt_region, bool is_module)
    >>> +static void __apply_alternatives(void *alt_region,  bool is_module,
    >>> +                 unsigned long *feature_mask)
    >>>   {
    >>>       struct alt_instr *alt;
    >>>       struct alt_region *region = alt_region;
    >>> @@ -165,6 +166,9 @@ static void __apply_alternatives(void *alt_region,
    >>> bool is_module)
    >>>       for (alt = region->begin; alt < region->end; alt++) {
    >>>           int nr_inst;
    >>>   +        if (!test_bit(alt->cpufeature, feature_mask))
    >>> +            continue;
    >>> +
    >>>           /* Use ARM64_CB_PATCH as an unconditional patch */
    >>>           if (alt->cpufeature < ARM64_CB_PATCH &&
    >>>               !cpus_have_cap(alt->cpufeature))
    >>> @@ -203,8 +207,11 @@ static void __apply_alternatives(void
    >>> *alt_region, bool is_module)
    >>>           __flush_icache_all();
    >>>           isb();
    >>>   -        /* We applied all that was available */
    >>> -        bitmap_copy(applied_alternatives, cpu_hwcaps, ARM64_NCAPS);
    >>> +        /* Ignore ARM64_CB bit from feature mask */
    >>> +        bitmap_or(applied_alternatives, applied_alternatives,
    >>> +              feature_mask, ARM64_NCAPS);
    >>> +        bitmap_and(applied_alternatives, applied_alternatives,
    >>> +               cpu_hwcaps, ARM64_NCAPS);
    >>>       }
    >>>   }
    >>>   @@ -225,8 +232,13 @@ static int __apply_alternatives_multi_stop(void
    >>> *unused)
    >>>               cpu_relax();
    >>>           isb();
    >>>       } else {
    >>> +        DECLARE_BITMAP(remaining_capabilities, ARM64_NPATCHABLE);
    >>> +
    >>> +        bitmap_complement(remaining_capabilities, boot_capabilities,
    >>> +                  ARM64_NPATCHABLE);
    >>> +
    >>>           BUG_ON(all_alternatives_applied);
    >>> -        __apply_alternatives(&region, false);
    >>> +        __apply_alternatives(&region, false, remaining_capabilities);
    >>>           /* Barriers provided by the cache flushing */
    >>>           WRITE_ONCE(all_alternatives_applied, 1);
    >>>       }
    >>> @@ -240,6 +252,24 @@ void __init apply_alternatives_all(void)
    >>>       stop_machine(__apply_alternatives_multi_stop, NULL,
    >>> cpu_online_mask);
    >>>   }
    >>>   +/*
    >>> + * 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 __init apply_boot_alternatives(void)
    >>> +{
    >>> +    struct alt_region region = {
    >>> +        .begin    = (struct alt_instr *)__alt_instructions,
    >>> +        .end    = (struct alt_instr *)__alt_instructions_end,
    >>> +    };
    >>> +
    >>> +    /* If called on non-boot cpu things could go wrong */
    >>> +    WARN_ON(smp_processor_id() != 0);
    >>> +
    >>> +    __apply_alternatives(&region, false, &boot_capabilities[0]);
    >>> +}
    >>> +
    >>>   #ifdef CONFIG_MODULES
    >>>   void apply_alternatives_module(void *start, size_t length)
    >>>   {
    >>> @@ -247,7 +277,10 @@ void apply_alternatives_module(void *start,
    >>> size_t length)
    >>>           .begin    = start,
    >>>           .end    = start + length,
    >>>       };
    >>> +    DECLARE_BITMAP(all_capabilities, ARM64_NPATCHABLE);
    >>> +
    >>> +    bitmap_fill(all_capabilities, ARM64_NPATCHABLE);
    >>>   -    __apply_alternatives(&region, true);
    >>> +    __apply_alternatives(&region, true, &all_capabilities[0]);
    >>>   }
    >>>   #endif
    >>> diff --git a/arch/arm64/kernel/cpufeature.c
    >>> b/arch/arm64/kernel/cpufeature.c
    >>> index 84fa5be..71c8d4f 100644
    >>> --- a/arch/arm64/kernel/cpufeature.c
    >>> +++ b/arch/arm64/kernel/cpufeature.c
    >>> @@ -54,6 +54,9 @@
    >>>   EXPORT_SYMBOL(cpu_hwcaps);
    >>>   static struct arm64_cpu_capabilities const __ro_after_init
    >>> *cpu_hwcaps_ptrs[ARM64_NCAPS];
    >>>   +/* Need also bit for ARM64_CB_PATCH */
    >>> +DECLARE_BITMAP(boot_capabilities, ARM64_NPATCHABLE);
    >>> +
    >>>   /*
    >>>    * Flag to indicate if we have computed the system wide
    >>>    * capabilities based on the boot time active CPUs. This
    >>> @@ -1672,6 +1675,9 @@ static void update_cpu_capabilities(u16 scope_mask)
    >>>           if (caps->desc)
    >>>               pr_info("detected: %s\n", caps->desc);
    >>>           cpus_set_cap(caps->capability);
    >>> +
    >>> +        if (caps->type & SCOPE_BOOT_CPU)
    >>
    >> You may want to do :
    >>         if (scope_mask & SCOPE_BOOT_CPU)
    >>
    >> for a tighter check to ensure this doesn't update the boot_capabilities
    >> after we have applied the boot_scope alternatives and miss applying the
    >> alternatives for those, should someone add a multi-scope (i.e
    >> SCOPE_BOOT_CPU and
    >> something else) capability (even by mistake).
    >>
    >
    > But a multi-scope capability containing SCOPE_BOOT_CPU should already
    > get updated for setup_boot_cpu_capabilities. Capabilities marked with
    > SCOPE_BOOT_CPU need to be enabled on the boot CPU or not at all.

    Yes, you're right. It is not normal to have multiple SCOPE for a "capability".
    But if someone comes with such a cap, we may miss handling this case. It is
    always better to be safer.

    >
    > Shouldn't the call to caps->matches() fail for a boot feature that was
    > not found on the boot cpu?
    >
    > Also, you made the opposite suggestion 4 version ago with a more
    > worrying scenario :) :
    > https://lkml.org/lkml/2018/5/25/208

    Ah, you're right. I missed that. We need the additional check as you mention
    below.

    >
    > Otherwise, if my assumption above is wrong, it means the check should
    > probably be:
    > if (caps->type & SCOPE_BOOT_CPU && scope_mask & SCOPE_BOOT_CPU)

    Yes, this is what we want.


    > But my current understanding is that we don't need that.
    >
    >> With that:
    >>
    >> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>


    Cheers
    Suzuki

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