[PATCH v3 4/4] ARM: Implement PAN for LPAE by TTBR0 page table walks disablement

[Florian Fainelli]

On 5/14/24 09:06, Geert Uytterhoeven wrote:
> On Tue, May 14, 2024 at 1:28 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>> On Tue, May 14, 2024 at 10:25 AM Ard Biesheuvel <ardb at kernel.org> wrote:
>>> On Tue, 14 May 2024 at 10:04, Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>>>> On Tue, May 14, 2024 at 9:59 AM Ard Biesheuvel <ardb at kernel.org> wrote:
>>>>> On Tue, 14 May 2024 at 09:46, Linus Walleij <linus.walleij at linaro.org> wrote:
>>>>>> On Tue, May 14, 2024 at 8:41 AM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>>>>>>
>>>>>>> I sent you a small initramfs by PM.
>>>>>>
>>>>>> Booted this just fine on Vexpress QEMU:
>>>>>>
>>>>>> Run /init as init process
>>>>>> sysctl: error: 'kernel.hotplug' is an unknown key
>>>>>>
>>>>>>
>>>>>> boot (Linux 6.9.0-rc1+, BusyBox v1.16.0.git, kexec-tools 2.0.1-git)
>>>>>> / # mount -t debugfs none /sys/kernel/debug
>>>>>> / # echo "ACCESS_USERSPACE" | cat >/sys/kernel/debug/provoke-crash/DIRECT
>>>>>> lkdtm: Performing direct entry ACCESS_USERSPACE
>>>>>> lkdtm: attempting bad read at 76fea000
>>>>>> 8<--- cut here ---
>>>>>> Unable to handle kernel paging request at virtual address 76fea000 when read
>>>>>> [76fea000] *pgd=82c93003, *pmd=82c94003, *pte=a00000811e2f5f
>>>>>> Internal error: Oops: 206 [#1] SMP ARM
>>>>>> CPU: 1 PID: 86 Comm: cat Not tainted 6.9.0-rc1+ #46
>>>>>> Hardware name: ARM-Versatile Express
>>>>>> PC is at lkdtm_ACCESS_USERSPACE+0xc0/0x138
>>>>>> LR is at lkdtm_ACCESS_USERSPACE+0xc0/0x138
>>>>>>
>>>>>> I'm starting to think it is something about different LPAE implementations here.
>>>>>
>>>>> I have built multi_v7_defconfig with the following enabled
>>>>>
>>>>> CONFIG_ARM_LPAE=y
>>>>> CONFIG_CPU_TTBR0_PAN=y
>>>>> CONFIG_LKDTM=y
>>>>>
>>>>> and the resulting kernel boots happily as a 32-bit VM running under a
>>>>> Rpi4 KVM host.
>>>>>
>>>>> Could someone post an actual .config that reproduces this? Rpi4 is
>>>>> A72, which both works and doesn't work in Florian's testing, so I'd be
>>>>> highly surprised if this is not a config issue.
>>>>
>>>> shmobile_defconfig with CONFIG_LPAE=y added failed for me before.
>>>>
>>>> Building multi_v7_defconfig with the above enabled...
>>
>> And that works, while shmobile_defconfig with the above does not!
>> So it is config-related.
> 
> I ran tools/testing/ktest/config-bisect.pl and found the "offending"
> config option: CONFIG_CC_OPTIMIZE_FOR_SIZE=y gives a broken
> kernel, CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE=y works.
> (verified by just enabling CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE in my
>   broken config)....

I second that, my working configuration has 
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE=y whereas my
broken one has CONFIG_CC_OPTIMIZE_FOR_SIZE=y, sure enough, changing the 
broken configuration to have CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE=y turns 
it into a "working" configuration on the Raspberry Pi 4B in AArch32 mode.

That makes more sense because BCM7211 and BCM2711 are exactly the same 
CPU, thanks for restoring sanity!
-- 
Florian