Re: [PATCH v7 00/23] arm64: Early CPU feature override, and applications to VHE, BTI and PAuth
From: Marc Zyngier
Date: Mon Feb 08 2021 - 10:13:37 EST
Hi Will,
On 2021-02-08 14:32, Will Deacon wrote:
Hi Marc,
On Mon, Feb 08, 2021 at 09:57:09AM +0000, Marc Zyngier wrote:
It recently came to light that there is a need to be able to override
some CPU features very early on, before the kernel is fully up and
running. The reasons for this range from specific feature support
(such as using Protected KVM on VHE HW, which is the main motivation
for this work) to errata workaround (a feature is broken on a CPU and
needs to be turned off, or rather not enabled).
This series tries to offer a limited framework for this kind of
problems, by allowing a set of options to be passed on the
command-line and altering the feature set that the cpufeature
subsystem exposes to the rest of the kernel. Note that this doesn't
change anything for code that directly uses the CPU ID registers.
I applied this locally, but I'm seeing consistent boot failure under
QEMU when
KASAN is enabled. I tried sprinkling some __no_sanitize_address
annotations
around (see below) but it didn't help. The culprit appears to be
early_fdt_map(), but looking a bit more closely, I'm really nervous
about the
way we call into C functions from __primary_switched. Remember -- this
code
runs _twice_ when KASLR is active: before and after the randomization.
This
also means that any memory writes the first time around can be lost due
to
the D-cache invalidation when (re-)creating the kernel page-tables.
Well, we already call into C functions with KASLR, and nothing explodes
with that, so I must be doing something else wrong.
I do have cache maintenance for the writes to the shadow registers, so
that
part should be fine. But I think I'm missing some cache maintenance
around
the FDT base itself, and I wonder what happens when we go around the
loop.
I'll chase this down now.
Thanks for the heads up.
M.
--
Jazz is not dead. It just smells funny...