Re: [PATCH] arm64: Enable MRS emulation early

From: Catalin Marinas
Date: Wed Oct 04 2017 - 07:10:52 EST

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@xxxxxxxxx>
> > Cc: James Morse <james.morse@xxxxxxx>
> > Cc: Dave Martin <Dave.martin@xxxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> 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.