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Xtensa selected neither fork primitive, so vfork() was simply absent. This wires it onto the two-primitive semantics. There is no assembly entry point and none is needed. Every exception entry already runs SPILL_ALL_WINDOWS, so the whole context of the calling thread is in its exception frame and copying its stack copies a complete frame chain. A flat build reaches that frame through SYS_save_context, issued inline so that the recorded stack pointer belongs to a frame that stays alive for the whole operation; a build with syscalls reaches it through xcp.sregs, recorded by xtensa_swint() for the duration of the call. The stack copy needs more than a relocated stack pointer here. A windowed ABI stores each frame's caller stack pointer absolutely, in the base save area below the frame, so a copy taken at a different address still names the parent throughout and the child's first retw would underflow onto the parent's stack. xtensa_fork_rebase() walks that chain and adds the relocation offset to each link. The copy also starts one base save area below the stack pointer rather than at it, because the frame the child resumes into keeps its caller's spilled a0-a3 there. Ported from the per-architecture work, reduced to the two primitives. Co-authored-by: Xiang Xiao <xiaoxiang781216@gmail.com> Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com> Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com> |
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