Re: [RFC PATCH 1/2] arm64/cpuinfo: Move init_cpu_features() ahead of setup.c::early_fixmap_init()

[Mark Rutland]

On Tue, Jan 26, 2021 at 01:57:13PM +0000, Will Deacon wrote:
> > @@ -297,16 +297,20 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
> >  	 */
> >  	arm64_use_ng_mappings = kaslr_requires_kpti();
> >  
> > -	early_fixmap_init();
> > -	early_ioremap_init();
> > -
> > -	setup_machine_fdt(__fdt_pointer);
> > -
> >  	/*
> >  	 * Initialise the static keys early as they may be enabled by the
> >  	 * cpufeature code and early parameters.
> >  	 */
> >  	jump_label_init();
> 
> I don't think your patch changes this, but afaict jump_label_init() uses
> per-cpu variables via cpus_read_lock(), yet we don't initialise our offset
> until later on. Any idea how that works?

We initialize the boot CPU's offset twice during boot, once before this
in smp_setup_processor_id(), and once afterwards in
smp_prepare_boot_cpu() since setup_per_cpu_areas() will allocate a new
region for CPU0.

IIUC per-cpu writes before smp_prepare_boot_cpu() are potentially dodgy
since they might be copied to other CPUs, but reads are all fine.

Mark.