[PATCH v3 0/4] ARM: vfp: Switch to C API to en/disable softirqs

[Guenter Roeck]

On 3/16/23 01:20, Ard Biesheuvel wrote:
> As it turns out, enabling or disabling softirqs from asm code is more
> tricky than it looks. Simply bumping the associated bit in preempt_count
> does the trick for uninstrumented kernels, but with lockdep or preempt
> debug enabled, we really need to call the C versions, as replicating
> their behavior in asm fully is intractible.
> 
> So let's rework the existing code a little bit so we can interpose a
> little C helper 'vfp_entry()' that disables softirqs before calling the
> existing vfpstate handling code in asm. Re-enabling softirqs from asm
> code is more straight-forward, as we can simply perform a tail call via
> a C routine that is guaranteed to be callable as a function.
> 
> However, since calling these APIs from asm code is still not ideal,
> let's reimplement the whole thing in C (patch #4)
> 
> 
> Changes since v2:
> - add Rbs and Tbs from Linus and Guenter (thanks!)
> - correct reference to local_bh_enable() vs __local_bh_enable_ip() in
>    commit log of patch #3
> - add patch that reworks the asm VFP exception handling entirely so the
>    bulk of it is implemented in C. This could be taken into v6.4, while
>    the  preceding patches are fixes for v6.3-rc
> 
> Cc: Frederic Weisbecker <frederic at kernel.org>
> Cc: Guenter Roeck <linux at roeck-us.net>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Linus Walleij <linus.walleij at linaro.org>
> Cc: Arnd Bergmann <arnd at arndb.de>
> Link: https://lore.kernel.org/all/ZBBYCSZUJOWBg1s8@localhost.localdomain/
> 
> Ard Biesheuvel (4):
>    ARM: vfp: Pass thread_info pointer to vfp_support_entry
>    ARM: vfp: Pass successful return address via register R3
>    ARM: vfp: Fix broken softirq handling with instrumentation enabled
>    ARM: vfp: Reimplement VFP exception entry in C code
> 
>   arch/arm/include/asm/assembler.h |  13 --
>   arch/arm/kernel/entry-armv.S     |  12 ++
>   arch/arm/vfp/Makefile            |   2 +-
>   arch/arm/vfp/entry.S             |  39 ----
>   arch/arm/vfp/vfp.h               |   1 +
>   arch/arm/vfp/vfphw.S             | 198 ++------------------
>   arch/arm/vfp/vfpmodule.c         | 125 +++++++++++-
>   7 files changed, 142 insertions(+), 248 deletions(-)
>   delete mode 100644 arch/arm/vfp/entry.S
> 

After applying this series, imx_v4_v5_defconfig and aspeed_g4_defconfig
(and maybe others) fail to build:

arm-linux-gnueabi-ld: arch/arm/kernel/entry-armv.o: in function `do_vfp':
(.entry.text+0x5a4): undefined reference to `vfp_entry'
make[1]: *** [scripts/Makefile.vmlinux:35: vmlinux] Error 1
make: *** [Makefile:1250: vmlinux] Error 2

Guenter