Re: [PATCH v12 2/7] virtio-pmem: Add virtio pmem driver

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 11 Jun 2019 22:07:57 +0530
Pankaj Gupta <pagupta@redhat.com> wrote:

> This patch adds virtio-pmem driver for KVM guest.
> 
> Guest reads the persistent memory range information from
> Qemu over VIRTIO and registers it on nvdimm_bus. It also
> creates a nd_region object with the persistent memory
> range information so that existing 'nvdimm/pmem' driver
> can reserve this into system memory map. This way
> 'virtio-pmem' driver uses existing functionality of pmem
> driver to register persistent memory compatible for DAX
> capable filesystems.
> 
> This also provides function to perform guest flush over
> VIRTIO from 'pmem' driver when userspace performs flush
> on DAX memory range.
> 
> Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
> Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> Acked-by: Jakub Staron <jstaron@google.com>
> Tested-by: Jakub Staron <jstaron@google.com>
> ---
>  drivers/nvdimm/Makefile          |   1 +
>  drivers/nvdimm/nd_virtio.c       | 124 +++++++++++++++++++++++++++++++
>  drivers/nvdimm/virtio_pmem.c     | 122 ++++++++++++++++++++++++++++++
>  drivers/nvdimm/virtio_pmem.h     |  55 ++++++++++++++
>  drivers/virtio/Kconfig           |  11 +++
>  include/uapi/linux/virtio_ids.h  |   1 +
>  include/uapi/linux/virtio_pmem.h |  35 +++++++++
>  7 files changed, 349 insertions(+)
>  create mode 100644 drivers/nvdimm/nd_virtio.c
>  create mode 100644 drivers/nvdimm/virtio_pmem.c
>  create mode 100644 drivers/nvdimm/virtio_pmem.h
>  create mode 100644 include/uapi/linux/virtio_pmem.h

Sorry about being late to the party; this one has been sitting in my
'to review' queue for far too long :(

(...)

> diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
> new file mode 100644
> index 000000000000..efc535723517
> --- /dev/null
> +++ b/drivers/nvdimm/nd_virtio.c
> @@ -0,0 +1,124 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * virtio_pmem.c: Virtio pmem Driver
> + *
> + * Discovers persistent memory range information
> + * from host and provides a virtio based flushing
> + * interface.
> + */
> +#include "virtio_pmem.h"
> +#include "nd.h"
> +
> + /* The interrupt handler */
> +void host_ack(struct virtqueue *vq)
> +{
> +	struct virtio_pmem *vpmem = vq->vdev->priv;
> +	struct virtio_pmem_request *req_data, *req_buf;
> +	unsigned long flags;
> +	unsigned int len;
> +
> +	spin_lock_irqsave(&vpmem->pmem_lock, flags);
> +	while ((req_data = virtqueue_get_buf(vq, &len)) != NULL) {
> +		req_data->done = true;
> +		wake_up(&req_data->host_acked);
> +
> +		if (!list_empty(&vpmem->req_list)) {
> +			req_buf = list_first_entry(&vpmem->req_list,
> +					struct virtio_pmem_request, list);
> +			req_buf->wq_buf_avail = true;
> +			wake_up(&req_buf->wq_buf);
> +			list_del(&req_buf->list);
> +		}
> +	}
> +	spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> +}
> +EXPORT_SYMBOL_GPL(host_ack);

Nit: 'host_ack' looks a bit generic for an exported function... would
'virtio_pmem_host_ack' maybe be better?

> +
> + /* The request submission function */
> +int virtio_pmem_flush(struct nd_region *nd_region)

I don't see an EXPORT_SYMBOL_GPL() for this function... should it get
one, or should it be made static?

> +{
> +	struct virtio_device *vdev = nd_region->provider_data;
> +	struct virtio_pmem *vpmem  = vdev->priv;
> +	struct virtio_pmem_request *req_data;
> +	struct scatterlist *sgs[2], sg, ret;
> +	unsigned long flags;
> +	int err, err1;
> +
> +	might_sleep();
> +	req_data = kmalloc(sizeof(*req_data), GFP_KERNEL);
> +	if (!req_data)
> +		return -ENOMEM;
> +
> +	req_data->done = false;
> +	init_waitqueue_head(&req_data->host_acked);
> +	init_waitqueue_head(&req_data->wq_buf);
> +	INIT_LIST_HEAD(&req_data->list);
> +	req_data->req.type = cpu_to_virtio32(vdev, VIRTIO_PMEM_REQ_TYPE_FLUSH);
> +	sg_init_one(&sg, &req_data->req, sizeof(req_data->req));
> +	sgs[0] = &sg;
> +	sg_init_one(&ret, &req_data->resp.ret, sizeof(req_data->resp));
> +	sgs[1] = &ret;
> +
> +	spin_lock_irqsave(&vpmem->pmem_lock, flags);
> +	 /*
> +	  * If virtqueue_add_sgs returns -ENOSPC then req_vq virtual
> +	  * queue does not have free descriptor. We add the request
> +	  * to req_list and wait for host_ack to wake us up when free
> +	  * slots are available.
> +	  */
> +	while ((err = virtqueue_add_sgs(vpmem->req_vq, sgs, 1, 1, req_data,
> +					GFP_ATOMIC)) == -ENOSPC) {
> +
> +		dev_err(&vdev->dev, "failed to send command to virtio pmem device, no free slots in the virtqueue\n");

Hm... by the comment above I would have thought that this is not really
an error, but rather a temporary condition? Maybe downgrade this to
dev_info()?

> +		req_data->wq_buf_avail = false;
> +		list_add_tail(&req_data->list, &vpmem->req_list);
> +		spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> +
> +		/* A host response results in "host_ack" getting called */
> +		wait_event(req_data->wq_buf, req_data->wq_buf_avail);
> +		spin_lock_irqsave(&vpmem->pmem_lock, flags);
> +	}
> +	err1 = virtqueue_kick(vpmem->req_vq);
> +	spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> +	/*
> +	 * virtqueue_add_sgs failed with error different than -ENOSPC, we can't
> +	 * do anything about that.
> +	 */

Does it make sense to kick if you couldn't add at all?

> +	if (err || !err1) {
> +		dev_info(&vdev->dev, "failed to send command to virtio pmem device\n");

If this is dev_info, I think the error above really should be dev_info
as well (and maybe also log the error value)?

> +		err = -EIO;
> +	} else {
> +		/* A host repsonse results in "host_ack" getting called */
> +		wait_event(req_data->host_acked, req_data->done);
> +		err = virtio32_to_cpu(vdev, req_data->resp.ret);
> +	}
> +
> +	kfree(req_data);
> +	return err;
> +};
> +
> +/* The asynchronous flush callback function */
> +int async_pmem_flush(struct nd_region *nd_region, struct bio *bio)
> +{
> +	/* Create child bio for asynchronous flush and chain with
> +	 * parent bio. Otherwise directly call nd_region flush.
> +	 */

Nit: The comment should start with an otherwise empty /* line.

> +	if (bio && bio->bi_iter.bi_sector != -1) {
> +		struct bio *child = bio_alloc(GFP_ATOMIC, 0);
> +
> +		if (!child)
> +			return -ENOMEM;
> +		bio_copy_dev(child, bio);
> +		child->bi_opf = REQ_PREFLUSH;
> +		child->bi_iter.bi_sector = -1;
> +		bio_chain(child, bio);
> +		submit_bio(child);
> +		return 0;
> +	}
> +	if (virtio_pmem_flush(nd_region))
> +		return -EIO;
> +
> +	return 0;
> +};
> +EXPORT_SYMBOL_GPL(async_pmem_flush);
> +MODULE_LICENSE("GPL");

(...)

I have only some more minor comments; on the whole, this looks good to
me.