[casaroli]

Summary

Adds the ucontext functions (getcontext(), setcontext(), makecontext(), swapcontext()) and include/ucontext.h, with an arm64 implementation that selects the new ARCH_HAVE_UCONTEXT. Programs that switch between their own stacks, such as coroutine libraries, need them.

NuttX has no ucontext today, and its sigaltstack() does not support an alternate stack (SS_ONSTACK fails with EINVAL), so the usual fallback of coroutine libraries does not work either: a program has no way to switch to a stack of its own. POSIX.1-2008 removed these functions (POSIX.1-2001 marked them obsolescent), but glibc, musl and FreeBSD keep them because programs still use them; apps/testing/ltp excludes the tests that include ucontext.h because NuttX lacks them.

A context holds the callee-saved registers, the stack pointer, the program counter, the arguments of makecontext(), the callee-saved FP registers when there is an FPU, and the signal mask. A context from makecontext() continues with uc_link when its function returns, or the task exits if uc_link is NULL.

QEMU, for example, needs them for its coroutines.

Impact

New header and functions, arm64 only. mcontext_t is in arch/arm64/include/irq.h, next to struct xcptcontext. Nothing else changes.

With CONFIG_TLS_ALIGNED (the default of the kernel and protected builds), user code finds its TLS from the stack pointer. Code on a makecontext() stack that uses TLS, for example exit() when the function returns with uc_link NULL, then reads a wrong address. #20723 removes this limit: user code reads the TLS pointer from TPIDR_EL0.

Testing

Builds: qemu-armv8a:nsh, qemu-armv8a:knsh, lm3s6965-ek:qemu-flat.

tools/checkpatch.sh -c -u -m -g passes.

A test program on qemu-armv8a:knsh (QEMU 11 with HVF on an Apple M4) with 8 checks: a getcontext()/setcontext() loop, two arguments with uc_link, a function with 10 arguments (two on the stack, negative values), chained contexts, 100 swapcontext() round trips, uc_sigmask, FP registers preserved across swapcontext(), and a task that exits when its context returns with uc_link NULL.

• This PR: 7 of 8 pass. The last check faults in exit() because of CONFIG_TLS_ALIGNED (see Impact).
• This PR on top of libs/libc/tls: Keep a copy of each __thread variable per thread. #20723: 8 of 8 pass.

ostest on qemu-armv8a:knsh and qemu-armv8a:nsh ends with ostest_main: Exiting with status 0.

[github-actions]

MemBrowse Memory Report

esp32-devkitc

• ROM: .flash.text +80 B (+0.1%, 126,280 B / 4,194,272 B, total: 3% used)
• irom0_0_seg: .flash.text +80 B (+0.1%, 90,476 B / 3,342,304 B, total: 3% used)

hifive1-revb

• flash: .text +4 B (+0.0%, 84,024 B / 4,194,304 B, total: 2% used)

qemu-armv8a

• Code: .rodata +56 B, .text.cp_handler +32 B, .text.get_primes_thread +80 B, .text.rr_test +116 B (+0.3%, 349,512 B)
• Data: -3 B (-0.0%, 77,687 B)

qemu-intel64

• Code: .text +565 B (+0.0%, 8,684,187 B)

rx65n-rsk2mb

• ROM: .text +304 B (+0.3%, 87,716 B / 2,097,152 B, total: 4% used)
  No memory changes detected for:
• arduino-mega2560
• mirtoo
• s698pm-dkit
• stm32-nucleo-f103rb