Method SubmitChain

src/device/virtio/virt_queue/split.hpp:460–513

* @brief 提交 Scatter-Gather 描述符链 * * 从空闲链表分配描述符，按顺序组装 readable（设备只读）和 * writable（设备可写） 缓冲区为描述符链，自动设置 NEXT 标志串联，并提交到 * Available Ring。 * * 描述符链顺序：[readable_0, ..., readable_N, writable_0, ..., writable_M] * - readable 部分：flags = kDescFNext（无 kDescFWrite） * - writable 部分：flags = kDescFNext | kDescFWrite * - 最后一个描述符清除 kDescFNext * * 调用者在调用此方法后仍需调用内存屏障 + Transport::Noti

   * @see 架构文档 §3 Scatter-Gather
   */
  [[nodiscard]] auto SubmitChain(const IoVec* readable, size_t readable_count,
                                 const IoVec* writable, size_t writable_count)
      -> Expected<uint16_t> {
    size_t total = readable_count + writable_count;
    if (total == 0) {
      return std::unexpected(Error{ErrorCode::kInvalidArgument});
    }
    if (num_free_ < static_cast<uint16_t>(total)) {
      return std::unexpected(Error{ErrorCode::kNoFreeDescriptors});
    }

    uint16_t head = free_head_;
    uint16_t prev_idx = 0xFFFF;

    for (size_t i = 0; i < readable_count; ++i) {
      uint16_t idx = free_head_;
      free_head_ = desc_[free_head_].next;
      --num_free_;

      desc_[idx].addr = readable[i].phys_addr;
      desc_[idx].len = static_cast<uint32_t>(readable[i].len);
      desc_[idx].flags = std::to_underlying(DescFlags::kDescFNext);

      if (prev_idx != 0xFFFF) {
        desc_[prev_idx].next = idx;
      }
      prev_idx = idx;
    }

    for (size_t i = 0; i < writable_count; ++i) {
      uint16_t idx = free_head_;
      free_head_ = desc_[free_head_].next;
      --num_free_;

      desc_[idx].addr = writable[i].phys_addr;
      desc_[idx].len = static_cast<uint32_t>(writable[i].len);
      desc_[idx].flags = std::to_underlying(DescFlags::kDescFNext) |
                         std::to_underlying(DescFlags::kDescFWrite);
      if (prev_idx != 0xFFFF) {
        desc_[prev_idx].next = idx;
      }
      prev_idx = idx;
    }

    desc_[prev_idx].flags =
        desc_[prev_idx].flags & ~std::to_underlying(DescFlags::kDescFNext);

    // 写屏障：确保描述符写入在 Available Ring 更新之前对设备可见
    cpu_io::Wmb();

    Submit(head);

    return head;
  }

  /**
   * @brief 释放整条描述符链