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

[James Morse]

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