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

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Tue, May 14, 2024 at 09:54:07AM -0700, Florian Fainelli wrote:
> On 5/14/24 09:06, Geert Uytterhoeven wrote:
> > On Tue, May 14, 2024 at 1:28 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > On Tue, May 14, 2024 at 10:25 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> > > > On Tue, 14 May 2024 at 10:04, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > > > On Tue, May 14, 2024 at 9:59 AM Ard Biesheuvel <ardb@kernel.org> wrote:
> > > > > > On Tue, 14 May 2024 at 09:46, Linus Walleij <linus.walleij@linaro.org> wrote:
> > > > > > > On Tue, May 14, 2024 at 8:41 AM Geert Uytterhoeven <geert@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!

I would imagine that the problem is cpu_set_ttbcr(). Please try adding
a "memory" clobber to the asm() instruction in there.

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