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Date: Thu, 10 Jan 2019 10:50:49 +0000
From: Julien Thierry <julien.thierry@....com>
To: Suzuki K Poulose <suzuki.poulose@....com>,
linux-arm-kernel@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org, daniel.thompson@...aro.org,
joel@...lfernandes.org, marc.zyngier@....com,
christoffer.dall@....com, james.morse@....com,
catalin.marinas@....com, will.deacon@....com, mark.rutland@....com
Subject: Re: [PATCH v8 15/26] arm64: alternative: Apply alternatives early in
boot process
On 08/01/2019 17:40, Suzuki K Poulose wrote:
>
>
> 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@...aro.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@...aro.org>
>>>> [julien.thierry@....com: rename to fit new cpufeature framework better,
>>>> apply BOOT_SCOPE feature early in boot]
>>>> Signed-off-by: Julien Thierry <julien.thierry@....com>
>>>> Cc: Catalin Marinas <catalin.marinas@....com>
>>>> Cc: Will Deacon <will.deacon@....com>
>>>> Cc: Christoffer Dall <christoffer.dall@....com>
>>>> Cc: Suzuki K Poulose <suzuki.poulose@....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(®ion, false);
>>>> + __apply_alternatives(®ion, 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(®ion, 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(®ion, true);
>>>> + __apply_alternatives(®ion, 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.
Yes you're right, the behaviour I was thinking of were the ID register,
where there views are altered for following CPUs when system wide
features are missing on the boot CPU. But ->matches() callbacks don't
only rely on ID registers.
I'll add that for the next version.
Thanks,
--
Julien Thierry
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