Re: [PATCH 2/3] sparc32: emulate casa on V8 CPUs

[Ramin Moussavi]

Hi Imre,

On Sun, Sep 27, 2026 at 02:58 AM Imre Kaloz wrote:
> so a C library can use one casa for its 32-bit atomics on any V8 part.

I sent a series covering part of the same ground, Magnus and Adrian
pointed me here -- I had not seen yours, so we worked on the same
problem in parallel:
https://lore.kernel.org/sparclinux/20261015085545.3246951-1-ramin.moussavi@xxxxxxxxx/

Your casa emulation looks like a good approach to me. What I would
still need is a way for a C library to find out whether the kernel
has it: a library built for v8 needs to keep its own fallback for
older kernels, and there casa is SIGILL. I see three ways a library
could tell:

- an AT_HWCAP bit, like HWCAP_SPARC_CASTRAP in Magnus' RFC but meaning
  "casa works, in hardware or emulated",
- a system call it can probe for ENOSYS, like patch 4 of my series,
- or trying casa once under a SIGILL handler at startup.

I know glibc does not want arch-specific atomics: on v8 the compiler
turns the atomic builtins into calls, and glibc would rather have
libgcc implement those with casa. But libgcc then faces the same
question, unless it simply requires a kernel with the emulation; and
uClibc-ng in my case keeps a fallback for older kernels. So from the
kernel side the question how a library can tell remains.

Regards,
Ramin