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

[Ard Biesheuvel]

On Fri, 17 Mar 2023 at 01:24, Guenter Roeck <linux at roeck-us.net> wrote:
>
> On 3/16/23 17:23, Guenter Roeck wrote:
> > 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
> >
>
> ... and reverting the last patch of the series fixes the problem.
>

Thanks Guenter,

It was a missing #ifdef CONFIG_VFP - I'll fix that up before I drop it
into the patch system.