[1/3] arch/xtensa: Provide vfork()

[casaroli]

Summary

Split from #19772, part 1 of 3, as asked in review.

It gives Xtensa vfork() and nothing else. No MMU, no address environment, no chip code.

up_fork(bool vfork) is the architecture half of the primitive introduced by #19562. The child gets its own stack holding a relocated copy of the part of the parent's that is in use, and resumes where the caller would have returned.

Two paths reach it. A flat build calls in directly, so the caller's registers are taken at the call. A build with system calls arrives through xtensa_swint(), which has already recorded the caller's frame in xcp.sregs; that frame is used instead, because the caller is on the far side of the boundary.

The windowed ABI needs care in one place. A stack pointer has a base save area 16 bytes below it holding the caller's spilled a0–a3, and the frame chain runs through it. SPILL_ALL_WINDOWS puts the register file into memory first, and the chain is then walked and rebased onto the child's copy, because a frame pointer that still points into the parent's stack would send the child back into memory it does not own.

Impact

Xtensa gains vfork(). Nothing else changes, and no board configuration changes.

fork() is not provided here. That needs an address environment and arrives in part three.

Testing

Host: macOS 15 on Apple Silicon, xtensa-esp32s3-elf-gcc and xtensa-esp32-elf-gcc, both 12.2.0.

Three boards ran ostest to the end.

board	module	core	configuration	result
ESP32-DevKitC V4	ESP32-D0WD-V3 rev 3.1	LX6	esp32-devkitc:ostest	vfork() passes, status 0
ESP32-S3-DevKitC	ESP32-S3-WROOM-1 N8R2, 8 MB, DIO	LX7	esp32s3-devkit:ostest	vfork() passes, status 0
ESP32-S3-DevKitC	ESP32-S3-WROOM-2 N32R8V, 32 MB, octal	LX7	esp32s3-devkit:ostest	vfork() passes, status 0

The 32 MB octal part also needs ESP32S3_FLASH_MODE_OCT and ESP32S3_SPI_FLASH_USE_32BIT_ADDRESS.

user_main: vfork() test
vfork_test: Started
vfork_test: Child 5 ran and exited before the parent resumed
ostest_main: Exiting with status 0

Both Xtensa cores that NuttX supports are covered, and the LX6 has no MMU at all — which is the case this part is meant to serve.

tools/checkpatch.sh -c -u -m -g reports no errors.

depends-on: apache/nuttx-apps/pull/3685

[github-actions]

MemBrowse Memory Report

esp32-devkitc

• ROM: .flash.text +32 B (+0.0%, 124,428 B / 4,194,272 B, total: 3% used)
• dram0_0_seg: .dram0.bss +16 B (+0.2%, 9,916 B / 180,736 B, total: 5% used)
• irom0_0_seg: .flash.text +32 B (+0.0%, 88,632 B / 3,342,304 B, total: 3% used)
  No memory changes detected for:
• arduino-mega2560
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb

[casaroli]

This work is now in #19798, together with the ESP32-S3 chip code and the user mode isolation that a kernel build needs.

Please review #19798 instead of this one.

This pull request stays open for reference, and it will be closed when #19798 is merged.

[github-actions]

🔗 Cross-repo PR dependencies

The read-only Build run reported the following dependent PR(s) and fetched head SHA(s):

• testing/ostest: Split the fork test into vfork and fork. nuttx-apps#3685 @ 2a43c6dae9

CI run: https://github.com/apache/nuttx/actions/runs/31531206452