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Add warn_on macro, uses the BUG/WARN feature (lib/bug.c) via assembly
for x86_64/arm64/riscv.
The current Rust code simply wraps BUG() macro but doesn't provide the
proper debug information. The BUG/WARN feature can only be used from
assembly.
This uses the assembly code exported by the C side via ARCH_WARN_ASM
macro. To avoid duplicating the assembly code, this approach follows
the same strategy as the static branch code: it generates the assembly
code for Rust using the C preprocessor at compile time.
Similarly, ARCH_WARN_REACHABLE is also used at compile time to
generate the assembly code; objtool's reachable annotation code. It's
used for only architectures that use objtool.
For now, Loongarch and arm just use a wrapper for WARN macro.
UML doesn't use the assembly BUG/WARN feature; just wrapping generic
BUG/WARN functions implemented in C works.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250502094537.231725-5-fujita.tomonori@gmail.com
[ Avoid evaluating the condition twice (a good idea in general,
but it also matches the C side). Simplify with `as_char_ptr()`
to avoid a cast. Cast to `ffi` integer types for
`warn_slowpath_fmt`. Avoid cast for `null()`. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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