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

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Message-ID: <20171004113207.GT3611@e103592.cambridge.arm.com>
Date:   Wed, 4 Oct 2017 12:32:07 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        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>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: Enable MRS emulation early

On Wed, Oct 04, 2017 at 12:10:40PM +0100, Catalin Marinas wrote:
> 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.

I don't think the hwcaps shouldn't change after entry to userspace,
so it really doesn't matter whether HWCAP_CPUID is set before or
after registration: for userspace it should all already have happened.


It looks to me like all initcalls are called in the same kernel thread
that execs the initramfs init process, before the exec.

So I still don't see how a built-in late initcall can not have been
called before entry to userspace.

The patch seems to demonstrate that I'm wrong though.
What am I missing?

Cheers
---Dave