Re: [PATCH v5 03/27] arm64: alternative: Apply alternatives early in boot process

From: Julien Thierry <julien.thierry@arm.com>
To: Daniel Thompson <daniel.thompson@linaro.org>
Cc: James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, joel@joelfernandes.org,
	marc.zyngier@arm.com, mark.rutland@arm.com,
	christoffer.dall@arm.com, catalin.marinas@arm.com,
	will.deacon@arm.com
Subject: Re: [PATCH v5 03/27] arm64: alternative: Apply alternatives early in boot process
Date: Tue, 18 Sep 2018 08:37:49 +0100	[thread overview]
Message-ID: <9505fbcc-796f-78bd-80b7-21d879c5e0bc@arm.com> (raw)
In-Reply-To: <20180917234400.ctq3w2u6fi5s5coj@holly.lan>

On 18/09/18 00:44, Daniel Thompson wrote:
> On Wed, Sep 12, 2018 at 05:49:09PM +0100, Julien Thierry wrote:
>>>> +	__apply_alternatives(&region, false, boot_capabilities);
>>>> +}
>>>> +
>>>>    #ifdef CONFIG_MODULES
>>>>    void apply_alternatives_module(void *start, size_t length)
>>>>    {
>>>
>>>> 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.
> 
> 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.
> 

When you say early version, do you mean the first ones you did? Because 
the one I picked up (v4 I believe) had a feature mask to select which 
ones to apply early and then which ones to exclude when applying the 
rest of the features.

But I admit I have not looked at the previous versions.

Cheers,

-- 
Julien Thierry