Date:   Wed, 12 Sep 2018 12:54:23 -0400
From:   Luiz Capitulino <lcapitulino@...hat.com>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        qemu-devel@...gnu.org, linux-nvdimm@...1.01.org, jack@...e.cz,
        stefanha@...hat.com, dan.j.williams@...el.com, riel@...riel.com,
        nilal@...hat.com, kwolf@...hat.com, pbonzini@...hat.com,
        ross.zwisler@...el.com, david@...hat.com,
        xiaoguangrong.eric@...il.com, hch@...radead.org, mst@...hat.com,
        niteshnarayanlal@...mail.com, imammedo@...hat.com,
        eblake@...hat.com
Subject: Re: [PATCH 3/3]  virtio-pmem: Add virtio pmem driver

On Fri, 31 Aug 2018 19:00:18 +0530
Pankaj Gupta <pagupta@...hat.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@...hat.com>
> ---
>  drivers/virtio/Kconfig           |   9 ++
>  drivers/virtio/Makefile          |   1 +
>  drivers/virtio/virtio_pmem.c     | 255 +++++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/virtio_ids.h  |   1 +
>  include/uapi/linux/virtio_pmem.h |  40 ++++++
>  5 files changed, 306 insertions(+)
>  create mode 100644 drivers/virtio/virtio_pmem.c
>  create mode 100644 include/uapi/linux/virtio_pmem.h
> 
> diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> index 3589764..a331e23 100644
> --- a/drivers/virtio/Kconfig
> +++ b/drivers/virtio/Kconfig
> @@ -42,6 +42,15 @@ config VIRTIO_PCI_LEGACY
>  
>  	  If unsure, say Y.
>  
> +config VIRTIO_PMEM
> +	tristate "Support for virtio pmem driver"
> +	depends on VIRTIO
> +	help
> +	This driver provides support for virtio based flushing interface
> +	for persistent memory range.
> +
> +	If unsure, say M.
> +
>  config VIRTIO_BALLOON
>  	tristate "Virtio balloon driver"
>  	depends on VIRTIO
> diff --git a/drivers/virtio/Makefile b/drivers/virtio/Makefile
> index 3a2b5c5..cbe91c6 100644
> --- a/drivers/virtio/Makefile
> +++ b/drivers/virtio/Makefile
> @@ -6,3 +6,4 @@ virtio_pci-y := virtio_pci_modern.o virtio_pci_common.o
>  virtio_pci-$(CONFIG_VIRTIO_PCI_LEGACY) += virtio_pci_legacy.o
>  obj-$(CONFIG_VIRTIO_BALLOON) += virtio_balloon.o
>  obj-$(CONFIG_VIRTIO_INPUT) += virtio_input.o
> +obj-$(CONFIG_VIRTIO_PMEM) += virtio_pmem.o
> diff --git a/drivers/virtio/virtio_pmem.c b/drivers/virtio/virtio_pmem.c
> new file mode 100644
> index 0000000..c22cc87
> --- /dev/null
> +++ b/drivers/virtio/virtio_pmem.c
> @@ -0,0 +1,255 @@
> +// 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 <linux/virtio.h>
> +#include <linux/module.h>
> +#include <linux/virtio_ids.h>
> +#include <linux/virtio_config.h>
> +#include <uapi/linux/virtio_pmem.h>
> +#include <linux/spinlock.h>
> +#include <linux/libnvdimm.h>
> +#include <linux/nd.h>
> +
> +struct virtio_pmem_request {
> +	/* Host return status corresponding to flush request */
> +	int ret;
> +
> +	/* command name*/
> +	char name[16];
> +
> +	/* Wait queue to process deferred work after ack from host */
> +	wait_queue_head_t host_acked;
> +	bool done;
> +
> +	/* Wait queue to process deferred work after virt queue buffer avail */
> +	wait_queue_head_t wq_buf;
> +	bool wq_buf_avail;
> +	struct list_head list;
> +};
> +
> +struct virtio_pmem {
> +	struct virtio_device *vdev;
> +
> +	/* Virtio pmem request queue */
> +	struct virtqueue *req_vq;
> +
> +	/* nvdimm bus registers virtio pmem device */
> +	struct nvdimm_bus *nvdimm_bus;
> +	struct nvdimm_bus_descriptor nd_desc;
> +
> +	/* List to store deferred work if virtqueue is full */
> +	struct list_head req_list;
> +
> +	/* Synchronize virtqueue data */
> +	spinlock_t pmem_lock;
> +
> +	/* Memory region information */
> +	uint64_t start;
> +	uint64_t size;
> +};
> +
> +static struct virtio_device_id id_table[] = {
> +	{ VIRTIO_ID_PMEM, VIRTIO_DEV_ANY_ID },
> +	{ 0 },
> +};
> +
> + /* The interrupt handler */
> +static void host_ack(struct virtqueue *vq)
> +{
> +	unsigned int len;
> +	unsigned long flags;
> +	struct virtio_pmem_request *req, *req_buf;
> +	struct virtio_pmem *vpmem = vq->vdev->priv;
> +
> +	spin_lock_irqsave(&vpmem->pmem_lock, flags);
> +	while ((req = virtqueue_get_buf(vq, &len)) != NULL) {
> +		req->done = true;
> +		wake_up(&req->host_acked);
> +
> +		if (!list_empty(&vpmem->req_list)) {
> +			req_buf = list_first_entry(&vpmem->req_list,
> +					struct virtio_pmem_request, list);
> +			list_del(&vpmem->req_list);
> +			req_buf->wq_buf_avail = true;
> +			wake_up(&req_buf->wq_buf);
> +		}
> +	}
> +	spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> +}
> + /* Initialize virt queue */
> +static int init_vq(struct virtio_pmem *vpmem)
> +{
> +	struct virtqueue *vq;
> +
> +	/* single vq */
> +	vpmem->req_vq = vq = virtio_find_single_vq(vpmem->vdev,
> +				host_ack, "flush_queue");
> +	if (IS_ERR(vq))
> +		return PTR_ERR(vq);
> +
> +	spin_lock_init(&vpmem->pmem_lock);
> +	INIT_LIST_HEAD(&vpmem->req_list);
> +
> +	return 0;
> +};
> +
> + /* The request submission function */
> +static int virtio_pmem_flush(struct nd_region *nd_region)
> +{
> +	int err;
> +	unsigned long flags;
> +	struct scatterlist *sgs[2], sg, ret;
> +	struct virtio_device *vdev =
> +		dev_to_virtio(nd_region->dev.parent->parent);
> +	struct virtio_pmem *vpmem = vdev->priv;

I'm missing a might_sleep() call in this function.

> +	struct virtio_pmem_request *req = kmalloc(sizeof(*req), GFP_KERNEL);
> +
> +	if (!req)
> +		return -ENOMEM;
> +
> +	req->done = req->wq_buf_avail = false;
> +	strcpy(req->name, "FLUSH");
> +	init_waitqueue_head(&req->host_acked);
> +	init_waitqueue_head(&req->wq_buf);
> +
> +	spin_lock_irqsave(&vpmem->pmem_lock, flags);
> +	sg_init_one(&sg, req->name, strlen(req->name));
> +	sgs[0] = &sg;
> +	sg_init_one(&ret, &req->ret, sizeof(req->ret));
> +	sgs[1] = &ret;

It seems that sg_init_one() is only setting fields, in this
case you can move spin_lock_irqsave() here.

> +	err = virtqueue_add_sgs(vpmem->req_vq, sgs, 1, 1, req, GFP_ATOMIC);
> +	if (err) {
> +		dev_err(&vdev->dev, "failed to send command to virtio pmem device\n");
> +
> +		list_add_tail(&vpmem->req_list, &req->list);
> +		spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> +
> +		/* When host has read buffer, this completes via host_ack */
> +		wait_event(req->wq_buf, req->wq_buf_avail);
> +		spin_lock_irqsave(&vpmem->pmem_lock, flags);

Is this error handling code assuming that at some point
virtqueue_add_sgs() will succeed for a different thread? If yes,
what happens if the assumption is false? That is, what happens if
virtqueue_add_sgs() never succeeds anymore?

Why not just return an error?

> +	}
> +	virtqueue_kick(vpmem->req_vq);
> +	spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> +
> +	/* When host has read buffer, this completes via host_ack */
> +	wait_event(req->host_acked, req->done);
> +	err = req->ret;

If I'm understanding the QEMU code correctly, you're returning EIO
from QEMU if fsync() fails. I think this is wrong, since we don't know
if EIO in QEMU will be the same EIO in the guest. One way to solve this
would be to return 0 for success and 1 for failure from QEMU, and let the
guest implementation pick its error code (for your implementation it
could be EIO).

> +	kfree(req);
> +
> +	return err;
> +};
> +EXPORT_SYMBOL_GPL(virtio_pmem_flush);
> +
> +static int virtio_pmem_probe(struct virtio_device *vdev)
> +{
> +	int err = 0;
> +	struct resource res;
> +	struct virtio_pmem *vpmem;
> +	struct nvdimm_bus *nvdimm_bus;
> +	struct nd_region_desc ndr_desc;
> +	int nid = dev_to_node(&vdev->dev);
> +	struct nd_region *nd_region;
> +
> +	if (!vdev->config->get) {
> +		dev_err(&vdev->dev, "%s failure: config disabled\n",
> +			__func__);
> +		return -EINVAL;
> +	}
> +
> +	vdev->priv = vpmem = devm_kzalloc(&vdev->dev, sizeof(*vpmem),
> +			GFP_KERNEL);
> +	if (!vpmem) {
> +		err = -ENOMEM;
> +		goto out_err;
> +	}
> +
> +	vpmem->vdev = vdev;
> +	err = init_vq(vpmem);
> +	if (err)
> +		goto out_err;
> +
> +	virtio_cread(vpmem->vdev, struct virtio_pmem_config,
> +			start, &vpmem->start);
> +	virtio_cread(vpmem->vdev, struct virtio_pmem_config,
> +			size, &vpmem->size);
> +
> +	res.start = vpmem->start;
> +	res.end   = vpmem->start + vpmem->size-1;
> +	vpmem->nd_desc.provider_name = "virtio-pmem";
> +	vpmem->nd_desc.module = THIS_MODULE;
> +
> +	vpmem->nvdimm_bus = nvdimm_bus = nvdimm_bus_register(&vdev->dev,
> +						&vpmem->nd_desc);
> +	if (!nvdimm_bus)
> +		goto out_vq;
> +
> +	dev_set_drvdata(&vdev->dev, nvdimm_bus);
> +	memset(&ndr_desc, 0, sizeof(ndr_desc));
> +
> +	ndr_desc.res = &res;
> +	ndr_desc.numa_node = nid;
> +	ndr_desc.flush = virtio_pmem_flush;
> +	set_bit(ND_REGION_PAGEMAP, &ndr_desc.flags);
> +	nd_region = nvdimm_pmem_region_create(nvdimm_bus, &ndr_desc);
> +
> +	if (!nd_region)
> +		goto out_nd;
> +
> +	//virtio_device_ready(vdev);
> +	return 0;
> +out_nd:
> +	err = -ENXIO;
> +	nvdimm_bus_unregister(nvdimm_bus);
> +out_vq:
> +	vdev->config->del_vqs(vdev);
> +out_err:
> +	dev_err(&vdev->dev, "failed to register virtio pmem memory\n");
> +	return err;
> +}
> +
> +static void virtio_pmem_remove(struct virtio_device *vdev)
> +{
> +	struct virtio_pmem *vpmem = vdev->priv;
> +	struct nvdimm_bus *nvdimm_bus = dev_get_drvdata(&vdev->dev);
> +
> +	nvdimm_bus_unregister(nvdimm_bus);
> +	vdev->config->del_vqs(vdev);
> +	kfree(vpmem);
> +}
> +
> +#ifdef CONFIG_PM_SLEEP
> +static int virtio_pmem_freeze(struct virtio_device *vdev)
> +{
> +	/* todo: handle freeze function */
> +	return -EPERM;
> +}
> +
> +static int virtio_pmem_restore(struct virtio_device *vdev)
> +{
> +	/* todo: handle restore function */
> +	return -EPERM;
> +}
> +#endif
> +
> +
> +static struct virtio_driver virtio_pmem_driver = {
> +	.driver.name		= KBUILD_MODNAME,
> +	.driver.owner		= THIS_MODULE,
> +	.id_table		= id_table,
> +	.probe			= virtio_pmem_probe,
> +	.remove			= virtio_pmem_remove,
> +#ifdef CONFIG_PM_SLEEP
> +	.freeze                 = virtio_pmem_freeze,
> +	.restore                = virtio_pmem_restore,
> +#endif
> +};
> +
> +module_virtio_driver(virtio_pmem_driver);
> +MODULE_DEVICE_TABLE(virtio, id_table);
> +MODULE_DESCRIPTION("Virtio pmem driver");
> +MODULE_LICENSE("GPL");
> diff --git a/include/uapi/linux/virtio_ids.h b/include/uapi/linux/virtio_ids.h
> index 6d5c3b2..3463895 100644
> --- a/include/uapi/linux/virtio_ids.h
> +++ b/include/uapi/linux/virtio_ids.h
> @@ -43,5 +43,6 @@
>  #define VIRTIO_ID_INPUT        18 /* virtio input */
>  #define VIRTIO_ID_VSOCK        19 /* virtio vsock transport */
>  #define VIRTIO_ID_CRYPTO       20 /* virtio crypto */
> +#define VIRTIO_ID_PMEM         25 /* virtio pmem */
>  
>  #endif /* _LINUX_VIRTIO_IDS_H */
> diff --git a/include/uapi/linux/virtio_pmem.h b/include/uapi/linux/virtio_pmem.h
> new file mode 100644
> index 0000000..c7c22a5
> --- /dev/null
> +++ b/include/uapi/linux/virtio_pmem.h
> @@ -0,0 +1,40 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * This header, excluding the #ifdef __KERNEL__ part, is BSD licensed so
> + * anyone can use the definitions to implement compatible drivers/servers:
> + *
> + *
> + * Redistribution and use in source and binary forms, with or without
> + * modification, are permitted provided that the following conditions
> + * are met:
> + * 1. Redistributions of source code must retain the above copyright
> + *    notice, this list of conditions and the following disclaimer.
> + * 2. Redistributions in binary form must reproduce the above copyright
> + *    notice, this list of conditions and the following disclaimer in the
> + *    documentation and/or other materials provided with the distribution.
> + * 3. Neither the name of IBM nor the names of its contributors
> + *    may be used to endorse or promote products derived from this software
> + *    without specific prior written permission.
> + * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS IS''
> + * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
> + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
> + * ARE DISCLAIMED.  IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE
> + * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
> + * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
> + * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
> + * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
> + * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
> + * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
> + * SUCH DAMAGE.
> + *
> + * Copyright (C) Red Hat, Inc., 2018-2019
> + * Copyright (C) Pankaj Gupta <pagupta@...hat.com>, 2018
> + */
> +#ifndef _UAPI_LINUX_VIRTIO_PMEM_H
> +#define _UAPI_LINUX_VIRTIO_PMEM_H
> +
> +struct virtio_pmem_config {
> +	__le64 start;
> +	__le64 size;
> +};
> +#endif

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