From: Ahmad Fatoum <ahmad@xxxxxx>
setjmp stores the registers comprising the execution context into a
jmp_buf and longjmp switches to that context and continues execution just
after the setjmp that allocated that jmp_buf.
setjmp/longjmp can have multiple uses:
- Handle return from firmware that clobbers registers. This is why we
already have a setjmp/longjmp ARM
- Do exception handling[1]
- Port over scripting language VMs that use sjlj for context switching
- Implement stackless coroutines in C.
These coroutines would be stackless, because jumping to a setjmp down the
call stack means that the code there will clobber the stack below it.
On resuming the coroutine, it will run with a stack changed in the
interim leading to undefined behavior.
There are ways around that without resorting to custom Assembly:
- Allocate a buffer on the scheduler's stack, so coroutine can
grow into them
-> Problem: exploits Undefined behavior
- Yield first time on scheduler stack, then patch jmp_buf to point at
another stack
-> Problem: Code switching stacks should not itself use the stack
- Construct a new jmp_buf with user-supplied return address and stack top:
int initjmp(jmp_buf jmp, void __noreturn (*func)(void), void *stack_top);
The latter could also be reused as a portable way of setting up a stack
in early init code.
We are going to implement green threads in later commits, so add a
symbol that can be selected to indicate architecture support for setjmp,
longjmp and the new initjmp.
[1]: https://www.spinics.net/lists/u-boot-v2/msg41322.html
Signed-off-by: Ahmad Fatoum <ahmad@xxxxxx>
---
common/Kconfig | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/common/Kconfig b/common/Kconfig
index edadcc9f4979..c0ff57bcdba4 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -28,6 +28,11 @@ config HAS_DMA
Drivers that depend on a DMA implementation can depend on this
config, so that you don't get a compilation error.
+config HAS_ARCH_SJLJ
+ bool
+ help
+ Architecture has support implemented for setjmp()/longjmp()/initjmp()
+
config GENERIC_GPIO
bool
--
2.29.2
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