Re: [PATCH 3/9] vhost-scsi: cache log buffer in I/O queue vhost_scsi_cmd

[Mike Christie]

On 2/7/25 12:41 PM, Dongli Zhang wrote:
> The vhost-scsi I/O queue uses vhost_scsi_cmd. Pre-allocate the log buffer
> during vhost_scsi_cmd allocation, and free it when vhost_scsi_cmd is
> reclaimed.
> 
> The cached log buffer will be uses in upcoming patches to log write
> descriptors for the I/O queue. The core idea is to cache the log in the
> per-command log buffer in the submission path, and use them to log write
> descriptors in the completion path.
> 
> Suggested-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> ---
>  drivers/vhost/scsi.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
> index ee2310555740..5e6221cbbe9e 100644
> --- a/drivers/vhost/scsi.c
> +++ b/drivers/vhost/scsi.c
> @@ -98,6 +98,11 @@ struct vhost_scsi_cmd {
>  	unsigned char tvc_cdb[VHOST_SCSI_MAX_CDB_SIZE];
>  	/* Sense buffer that will be mapped into outgoing status */
>  	unsigned char tvc_sense_buf[TRANSPORT_SENSE_BUFFER];
> +	/*
> +	 * Dirty write descriptors of this command.
> +	 */
> +	struct vhost_log *tvc_log;
> +	unsigned int tvc_log_num;
>  	/* Completed commands list, serviced from vhost worker thread */
>  	struct llist_node tvc_completion_list;
>  	/* Used to track inflight cmd */
> @@ -619,6 +624,7 @@ vhost_scsi_get_cmd(struct vhost_virtqueue *vq, struct vhost_scsi_tpg *tpg,
>  	struct vhost_scsi_nexus *tv_nexus;
>  	struct scatterlist *sg, *prot_sg;
>  	struct iovec *tvc_resp_iov;
> +	struct vhost_log *log;
>  	struct page **pages;
>  	int tag;
>  
> @@ -639,6 +645,7 @@ vhost_scsi_get_cmd(struct vhost_virtqueue *vq, struct vhost_scsi_tpg *tpg,
>  	prot_sg = cmd->tvc_prot_sgl;
>  	pages = cmd->tvc_upages;
>  	tvc_resp_iov = cmd->tvc_resp_iov;
> +	log = cmd->tvc_log;
>  	memset(cmd, 0, sizeof(*cmd));
>  	cmd->tvc_sgl = sg;
>  	cmd->tvc_prot_sgl = prot_sg;
> @@ -652,6 +659,7 @@ vhost_scsi_get_cmd(struct vhost_virtqueue *vq, struct vhost_scsi_tpg *tpg,
>  	cmd->tvc_nexus = tv_nexus;
>  	cmd->inflight = vhost_scsi_get_inflight(vq);
>  	cmd->tvc_resp_iov = tvc_resp_iov;
> +	cmd->tvc_log = log;
>  
>  	memcpy(cmd->tvc_cdb, cdb, VHOST_SCSI_MAX_CDB_SIZE);
>  
> @@ -1604,6 +1612,7 @@ static void vhost_scsi_destroy_vq_cmds(struct vhost_virtqueue *vq)
>  		kfree(tv_cmd->tvc_prot_sgl);
>  		kfree(tv_cmd->tvc_upages);
>  		kfree(tv_cmd->tvc_resp_iov);
> +		kfree(tv_cmd->tvc_log);
>  	}
>  
>  	sbitmap_free(&svq->scsi_tags);
> @@ -1666,6 +1675,18 @@ static int vhost_scsi_setup_vq_cmds(struct vhost_virtqueue *vq, int max_cmds)
>  			pr_err("Unable to allocate tv_cmd->tvc_prot_sgl\n");
>  			goto out;
>  		}
> +
> +		/*
> +		 * tv_cmd->tvc_log and vq->log need to have the same max
> +		 * length.
> +		 */
> +		tv_cmd->tvc_log = kcalloc(vq->dev->iov_limit,
> +					  sizeof(struct vhost_log),
> +					  GFP_KERNEL);

VHOST_F_LOG_ALL is normally set when the migration starts right?

I mean it's done before the initial setup when the above code is run so
is it possible to do the allocation when vhost_scsi_set_features is passed
VHOST_F_LOG_ALL? We then don't allocate a bunch of mem for a feature that
may never be used.