Robert Allan JamesandClaude Sonnet 5
f790d0995e
Third stage of the preemptive context-switching plan. The real save/restore switch mechanism now exists -- the first time anything has ever executed on a VM's own native stack (Stage 1 allocated them, unused). New sk_vm_switch_to() (switch.S, one per arch) is an ordinary function call, not an interrupt -- so unlike Stage 0's trap frame, the ABI already covers every caller-saved register; only the callee-saved set needs explicit save/restore (amd64: rbx/rbp/r12-r15, no FP at all since SysV has no callee-saved XMM; aarch64: x19-x28/x29/x30 + d8-d15; riscv64: s0-s11/ra + fs0-fs11, FS-gated like Stage 0 but read once and reused for both halves within one call, since FS is genuine global CPU state, not part of what's switched). A sibling sk_vm_switch_prime() in the same file builds the synthetic first-entry frame, kept in assembly so the layout can never drift out of sync with sk_vm_switch_to() itself. New switch.c/switch.h: sk_vm_context_switch(from, to) handles first-entry priming vs. resuming a parked context, and updates registry state (new VM_STATE_SWITCHED_OUT, distinct from VM_STATE_STOPPED -- STOPPED means no live frame, this means the opposite). sk_vm_switch_entry() is the minimal permanent trampoline every freshly-entered VM lands in: no production behavior defined yet, so it just yields straight back to whoever switched to it, forever. Closes the confirmed unguarded-KILL UAF found during planning: capsule_vm_kill(), mama_word_kill(), and capsule_vm_kill_all_nonmama() all now refuse (or silently leak rather than free, on the cold-restart path where arch_cold_reset() wipes everything immediately after anyway) tearing down a switched-out VM. Side effect found, not built on purpose: the existing MSG-TICK idle-pump already filters on VM_STATE_LIVE, so it automatically stopped dispatching into a switched-out VM with zero changes needed there. Verified via a temporary SWITCH-TEST probe (boot-triggered, since nothing can type interactively into a foreground-only QEMU session) that round-tripped a sentinel through 5 real Hera<->Hermes switches on all 3 architectures: 5/5 rounds, 0 failures, clean continuation to ok>. Probe fully reverted after capture; kernel_main.c shows zero diff. Also: Makefile.starkernel's LOADER_EXTRA_SRCS/LOADER_ASM needed the new files added explicitly (this project's "loader" PE binary is the full running kernel, not a thin bootstrap stage), and aarch64's switch.S needed the same #ifndef _WIN32 guard around .hidden that isr.S already carries (aarch64's loader assembles via clang targeting a PE/COFF target with no .hidden equivalent) -- caught by a build failure, fixed. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_016UNhH1mhi52i6Qihh7ZV5S
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