Re: [PATCH v2 2/2] arm64: cpufeature: Pretend that Apple A10 family does not support 32-bit EL0

Catalin Marinas 於 2024/9/16 晚上11:30 寫道:

On Mon, Sep 16, 2024 at 09:41:12PM +0800, Nick Chan wrote:

quoted

On 9/9/2024 17:10, Nick Chan wrote:

quoted

The Apple A10 family consists of physical performance and efficiency
cores, and only one of them can be active at a given time depending on
the current p-state. However, only the performance cores can execute
32-bit EL0. This results in logical cores that can only execute 32-bit
EL0 in high p-states.

Further research shows that the MPIDR_EL1 values between the two core
types are different. And whether the two core type have any extra
differences is anyone's guess right now. So far, nothing seems to break
horribly without special workarounds for the MPIDR value (with cpufreq
enabled downstream) as:
1. There are no KVM, GIC, ACPI, PSCI or cpuidle
2. All CPUs switch P-mode and E-mode together

However, all of this is broken enough that this piece of code should go
into arch/arm64/kernel/cpu_errata.c, and also generate a
TAINT_CPU_OUT_OF_SPEC for these cursed CPUs.

I wouldn't carry any additional logic in the kernel for such
configuration (long time ago Arm had something similar, the big.LITTLE
switcher, but the CPUs were fairly similar from a feature perspective).

This is fine from a functionality perspective, currently nothing that
accesses MPIDR after boot is used on A10(X). However, it does not sound
right either to not note that the kernel is running on a cursed CPU.

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Trying to support 32-bit EL0 on a CPU that can only execute it in certain
states is a bad idea. The A10 family only supports 16KB page size anyway
so many AArch32 executables won't run anyways. Pretend that it does not
support 32-bit EL0 at all.

CONFIG_COMPAT depends on ARM64_4K_PAGES || EXPERT. Do we really need
these patches in case one enables EXPERT and tries to run 32-bit
binaries that never ran on 16K pages before?

The worst thing that can happen is the 32-bit process crashes with illegal
instruction, the kernel will still be fine.

Nick Chan