Honor futex_waitv clockid and wake without polling

[jserv]

futex_waitv (SYS 449) is SYSCALL_DEFINE5 in the kernel: (waiters, nr_futexes, flags, timeout, clockid). The dispatch wrapper sc_futex_waitv was forwarding only x0..x3, dropping x4. The implementation hardcoded a CLOCK_MONOTONIC->CLOCK_REALTIME deadline conversion, so a guest asking for CLOCK_REALTIME got monotonic semantics regardless of what it passed. Wire x4 through and give sys_futex_waitv a clockid parameter. Validate against CLOCK_REALTIME or CLOCK_MONOTONIC when timeout is non-NULL (Linux returns EINVAL otherwise) and branch the absolute-deadline conversion on clockid.

The pre-existing waitv loop slept on a private cond that no wake site ever signalled; forward progress relied on a 50ms poll. Add optional group_lock / group_cond pointers to futex_waiter_t plus a futex_waiter_notify_group helper. Each wake site (futex_wake, futex_requeue, futex_wake_op both passes, futex_unlock_pi) calls it after marking woken=1 under the bucket lock. waitv blocks on shared.cond directly; the loop's bounded sleep is now 500ms, kept only so exit_group and timeout edges are still observed when no signal arrives.

Lock ordering is bucket -> group_lock; futex_waiter_notify_group is only called by wake sites that already hold the bucket lock, and the waitv thread never holds shared.lock while taking a bucket lock. Stack lifetime of shared.lock and shared.cond is protected by the bucket-lock pairing in waitv_unlink, which synchronizes with every wake's notify_group before the destroys run.

Locked in by tests/test-futex-waitv.c (19 cases): single-element wake returning index 0, multi-element wake returning the woken index, EAGAIN on stale val, eight EINVAL paths covering nr=0, nr>128, top-level flags, reserved!=0, element flags reserved bits, size!=U32, malformed CLOCK_REALTIME / CLOCK_MONOTONIC nsec, unaligned uaddr, NULL waiters_gva, bad clockid with timeout; two EFAULT paths via PROT_NONE pages; the inclusive nr_futexes==128 boundary; and ETIMEDOUT under both CLOCK_MONOTONIC and CLOCK_REALTIME deadlines. The CLOCK_REALTIME case exists specifically to catch a regression of the dropped-clockid bug. Verified against Linux ground truth via tests/qemu-runner.sh; all 19 cases match.

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Summary by cubic

Fixes futex_waitv to honor the clockid argument and switches waitv to signal-driven wakeups, removing the 50ms poll and matching Linux behavior. This improves correctness (timeouts) and reduces wake latency.

• Bug Fixes

  • Forward clockid to sys_futex_waitv; support CLOCK_REALTIME and CLOCK_MONOTONIC, return EINVAL for others when a timeout is provided.
  • Convert absolute deadlines based on the selected clock; validate timespec (EINVAL) and faulting pointers (EFAULT) to mirror Linux.
  • Validate per-element fields (reserved bits, size U32, 4-byte alignment), reject NULL waiters, and enforce 1..128 nr_futexes.

• Refactors

  • Replace polling with group-condition signaling from all wake sites to unblock waitv; keep a 500ms bounded sleep to observe exit/timeouts.
  • Strengthen waiter unlinking to handle requeue races and enforce lock order (bucket -> group lock).
  • Add tests/test-futex-waitv (19 cases) with a Makefile rule using -lpthread, and register it in tests/manifest.txt.

^{Written for commit 1423b4a. Summary will update on new commits.}

[cubic-dev-ai]

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