Re: [PATCH v5 7/8] hw/nvme: add basic live migration support

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Mar 17, 2026 at 11:27:07AM +0100, Alexander Mikhalitsyn wrote:
> +    /* wait when all in-flight IO requests (except NVME_ADM_CMD_ASYNC_EV_REQ) are processed */
> +    for (i = 0; i < n->num_queues; i++) {
> +        NvmeRequest *req;
> +        NvmeSQueue *sq = n->sq[i];
> +
> +        if (!sq)
> +            continue;
> +
> +        trace_pci_nvme_pre_save_sq_out_req_drain_wait(n, i, sq->head, sq->tail, sq->size);
> +
> +wait_out_reqs:
> +        QTAILQ_FOREACH(req, &sq->out_req_list, entry) {
> +            if (req->cmd.opcode != NVME_ADM_CMD_ASYNC_EV_REQ) {
> +                cpu_relax();
> +                goto wait_out_reqs;
> +            }
> +        }
> +
> +        trace_pci_nvme_pre_save_sq_out_req_drain_wait_end(n, i, sq->head, sq->tail);
> +    }

Emulated storage controllers usually do not drain requests themselves.
They rely on core migration code (e.g. migration_completion_precopy())
to stop vCPUs and call bdrv_drain_all_begin/end() to quiesce I/O. Why
does NVMe busy wait for requests here?

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