linux-kernel - Re: [PATCH] arm64: Enable MRS emulation early

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Date:   Wed, 4 Oct 2017 12:10:40 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        matwey.kornilov@...il.com, James Morse <james.morse@....com>,
        Dave Martin <Dave.martin@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: Enable MRS emulation early

On Wed, Oct 04, 2017 at 11:14:26AM +0100, Mark Rutland wrote:
> On Wed, Oct 04, 2017 at 10:48:05AM +0100, Suzuki K Poulose wrote:
> > Make sure the MRS emulation is enabled early enough, such that the
> > early userspace applications (e.g, those run from initrd) could
> > use the facility without crashing them.
> > 
> > Fixes: commit 77c97b4ee2129 ("arm64: cpufeature: Expose CPUID registers by emulation")
> > Reported-by: Matwey V. Kornilov <matwey.kornilov@...il.com>
> > Cc: James Morse <james.morse@....com>
> > Cc: Dave Martin <Dave.martin@....com>
> > Cc: Catalin Marinas <catalin.marinas@....com>
> > Cc: Will Deacon <will.deacon@....com>
> > Cc: stable@...r.kernel.org
> > Cc: Mark Rutland <mark.rutland@....com>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> 
> This looks sensible, but shouldn't we do the same for other
> late_inicalls can affect initrd userspace?
> 
> e.g. armv8_deprecated_init, fpsimd_init, sys_reg_genericv8_init?

I think we should, though not all of them are concerned with the user
code. For example, fpsimd_init() takes care of the pm/hotplug aspect and
nothing to do with user space. That said, making it core_initcall() is
probably not a bad thing (just a statement that it is concerned with the
core initialisation), as long as all the other infrastructure it
registers with is up.

For Suzuki's patch, I was thinking of enabling emulation before we
register the HWCAP_CPUID bit (setup_elf_hwcaps). However, that means we
have to bring it before smp_cpus_done(). It's not really worth it as we
don't expect any user space at that point.

-- 
Catalin