arch/x86_64: Implement up_addrenv_fork() and provide POSIX fork() - #19776
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arch/x86_64: Implement up_addrenv_fork() and provide POSIX fork()#19776casaroli wants to merge 2 commits into
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Duplicate an address environment into freshly allocated pages mapped at the same virtual addresses, which is what POSIX fork() is built on. x86_64_fork_syscall() then lets the child run at the parent's stack addresses. A pointer to a stack local taken before fork() must name the same object in the child that it named in the parent, so the child adopts the parent's stack geometry rather than being given a relocated copy; the parent's stack is already in the duplicate, at the parent's address, with its contents. That shows up as a zero offset, which also means the copy would have the same source and destination, so both the copy and the frame-pointer relocation are skipped. Build-verified on qemu-intel64:knsh_romfs. NuttX on qemu-intel64 requires tsc-deadline and pcid, which TCG does not implement, so it cannot be run on this host. Assisted-by: Claude Code:claude-opus-5 Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
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Review of apache#19772 asked for this shape, and it applies to every architecture in the series. ARCH_HAVE_FORK described when it was available from inside its own definition, which put the per-architecture condition somewhere nobody looks. The architecture now says so itself. The condition repeats the ARCH_ADDRENV dependency rather than relying on it, because a select bypasses depends on: without that repetition an architecture could offer fork() where there is no address environment to duplicate. Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com> Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
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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fork()was withdrawn from every architecture by #19562, and each architecture restores it with the correct behaviour.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 inxcp.sregsandx86_64_fork()chooses betweenx86_64_fork_syscall()andx86_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_FORKkeepsdepends 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-gcc16.1.0. There is no hardware x86 acceleration on this host, so this ran under TCG.qemu-intel64:knsh_romfs, a kernel build:tools/checkpatch.sh -c -u -m -ggives no errors.Two notes for anyone repeating this under TCG.
-cpu maxis the flag that matters, and a named Intel model is much slower. Three configuration changes are necessary as well: disableARCH_INTEL64_HAVE_PCIDandARCH_INTEL64_TSC_DEADLINE, and enableARCH_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_romfsand run/system/bin/ostest. The fork tests are the first output.