Messages in this thread | | | Subject | Re: [PATCH v5 03/27] arm64: alternative: Apply alternatives early in boot process | From | James Morse <> | Date | Tue, 18 Sep 2018 18:47:43 +0100 |
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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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