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

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Date:   Tue, 26 Jan 2021 14:11:48 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Jia He <justin.he@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Anshuman Khandual <anshuman.khandual@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Richard Henderson <richard.henderson@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Steven Price <steven.price@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Gavin Shan <gshan@...hat.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Mark Brown <broonie@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Cristian Marussi <cristian.marussi@....com>
Subject: Re: [RFC PATCH 1/2] arm64/cpuinfo: Move init_cpu_features() ahead of
 setup.c::early_fixmap_init()

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.