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    SubjectRe: [PATCH v5 03/27] arm64: alternative: Apply alternatives early in boot process
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    Hi Daniel, Julien,

    On 09/18/2018 12:44 AM, Daniel Thompson wrote:
    > On Wed, Sep 12, 2018 at 05:49:09PM +0100, Julien Thierry wrote:
    >>>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
    >>>> index 3bc1c8b..0d1e41e 100644
    >>>> --- a/arch/arm64/kernel/cpufeature.c
    >>>> +++ b/arch/arm64/kernel/cpufeature.c
    >>>> @@ -52,6 +52,8 @@
    >>>> DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
    >>>> EXPORT_SYMBOL(cpu_hwcaps);
    >>>> +unsigned long boot_capabilities;
    >>>> +
    >>>> /*
    >>>> * Flag to indicate if we have computed the system wide
    >>>> * capabilities based on the boot time active CPUs. This
    >>>> @@ -1375,6 +1377,9 @@ static void __update_cpu_capabilities(const struct arm64_cpu_capabilities *caps,
    >>>> if (!cpus_have_cap(caps->capability) && caps->desc)
    >>>> pr_info("%s %s\n", info, caps->desc);
    >>>> cpus_set_cap(caps->capability);
    >>>
    >>> Hmm, the bitmap behind cpus_set_cap() is what cpus_have_cap() in
    >>> __apply_alternatives() looks at. If you had a call to __apply_alternatives after
    >>> update_cpu_capabilities(SCOPE_BOOT_CPU), but before any others, it would only
    >>> apply those alternatives...
    >>>
    >>> (I don't think there is a problem re-applying the same alternative, but I
    >>> haven't checked).

    >> Interesting idea. If someone can confirm that patching alternatives twice is
    >> safe, I think it would make things simpler.

    Sounds good, I think we need to avoid adding a limit to the number of caps.

    The extra-work is inefficient, but if it saves merging those lists as part of this
    series its probably fine. (we only do this stuff once during boot)



    > Early versions of this patch applied the alternatives twice. I never
    > noticed any problems with double patching (second time round it will
    > write out code that is identical to what is already there so it is
    > merely inefficient rather than unsafe.

    For the regular kind, I agree. But we've recently grown some fancy dynamic patching
    where the code is generated at runtime, instead of swapping in an alternative
    sequence. Details in commit dea5e2a4 ("arm64: alternatives: Add dynamic patching
    feature"). Its unlikely we would ever apply these twice as they can't have a scope,
    ... and they all look safe.


    Thanks,

    James

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