casaroli opened a new pull request, #19776:
URL: https://github.com/apache/nuttx/pull/19776
> **Draft.** The evidence is emulation only. I keep this as a draft until
`fork()` runs on x86_64 hardware.
## Summary
`fork()` was withdrawn from every architecture by #19562, and each
architecture restores it with the correct behaviour. This pull request restores
it for x86_64.
`up_addrenv_fork()` duplicates an address environment into freshly allocated
pages mapped at the same virtual addresses. The text, data and heap regions of
the source are walked one page at a time and copied into fresh pages hung off
the page tables of the child.
The saved-syscall-frame half is already in master. #19562 added it, so
`x86_64_syscall()` stores the exception frame of the caller in `xcp.sregs` and
`x86_64_fork()` chooses between `x86_64_fork_syscall()` and
`x86_64_fork_direct()`.
## Impact
The change is specific to x86_64. It adds capability and removes none.
`fork()` becomes available on x86_64 outside a protected build.
`ARCH_HAVE_FORK` keeps `depends on ARCH_ADDRENV`, so a configuration without
address environments is unaffected.
`vfork()` does not change. No board configuration changes.
## Testing
Emulation only. QEMU 11.0.3 on macOS 15 with Apple Silicon, `x86_64-elf-gcc`
16.1.0. There is no hardware x86 acceleration on this host, so this ran under
TCG.
`qemu-intel64:knsh_romfs`, a kernel build:
```
qemu-system-x86_64 -machine pc,hpet=on -cpu max -m 2G \
-kernel nuttx -nographic -no-reboot -net none
```
```
user_main: vfork() test
vfork_test: Child 6 ran and exited before the parent resumed
user_main: fork() test
fork_test: Child running independently (child)
fork_test: Parent and child had independent memory
ostest_main: Exiting with status 0
```
`tools/checkpatch.sh -c -u -m -g` gives no errors.
Two notes for anyone repeating this under TCG. `-cpu max` is the flag that
matters, and a named Intel model is much slower. Three configuration changes
are necessary as well: disable `ARCH_INTEL64_HAVE_PCID` and
`ARCH_INTEL64_TSC_DEADLINE`, and enable `ARCH_INTEL64_HPET_ALARM`. Without them
the console stops after SeaBIOS and looks like a hang.
### What is not tested
No x86_64 hardware, and no KVM. This is the reason for the draft status.
The protected build is excluded by the Kconfig condition and is not tested.
**A report from real x86_64 hardware is welcome.** Take this branch with
apache/nuttx-apps#3685, build `qemu-intel64:knsh_romfs` and run
`/system/bin/ostest`. The fork tests are the first output.
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