Re: [Qemu-devel] [RFC v3 2/2] virtio-pmem: Add virtio pmem driver

[Luiz Capitulino]

On Fri, 13 Jul 2018 13:22:31 +0530
Pankaj Gupta <address@hidden> 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 <address@hidden>
> ---
>  drivers/virtio/Kconfig           |   9 ++
>  drivers/virtio/Makefile          |   1 +
>  drivers/virtio/virtio_pmem.c     | 190 
> +++++++++++++++++++++++++++++++++++++++
>  include/linux/virtio_pmem.h      |  44 +++++++++
>  include/uapi/linux/virtio_ids.h  |   1 +
>  include/uapi/linux/virtio_pmem.h |  40 +++++++++
>  6 files changed, 285 insertions(+)
>  create mode 100644 drivers/virtio/virtio_pmem.c
>  create mode 100644 include/linux/virtio_pmem.h
>  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..6200b5e
> --- /dev/null
> +++ b/drivers/virtio/virtio_pmem.c
> @@ -0,0 +1,190 @@
> +// 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_pmem.h>
> +
> +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;
> +     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->acked);
> +     }
> +     spin_unlock_irqrestore(&vpmem->pmem_lock, flags);

Honest question: why do you need to disable interrupts here?

> +}
> + /* 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);
> +
> +     return 0;
> +};
> +
> + /* The request submission function */
> +static int virtio_pmem_flush(struct device *dev)
> +{
> +     int err;
> +     unsigned long flags;
> +     struct scatterlist *sgs[2], sg, ret;
> +     struct virtio_device *vdev = dev_to_virtio(dev->parent->parent);
> +     struct virtio_pmem *vpmem = vdev->priv;
> +     struct virtio_pmem_request *req = kmalloc(sizeof(*req), GFP_KERNEL);

Not checking kmalloc() return.

> +
> +     req->done = false;
> +     init_waitqueue_head(&req->acked);
> +     spin_lock_irqsave(&vpmem->pmem_lock, flags);

Why do you need spin_lock_irqsave()? There are two points consider:

1. Will virtio_pmem_flush() ever be called with interrupts disabled?
   If yes, then it's broken since you should be using GFP_ATOMIC in the
   kmalloc() call and you can't call wait_event()

2. If virtio_pmem_flush() is never called with interrupts disabled, do
   you really need to disable interrupts? If yes, why?

Another point to consider is whether or not virtio_pmem_flush()
can be called from atomic context. nvdimm_flush() itself is called
from a few atomic sites, but I can't tell if virtio_pmem_flush()
will ever be called from those sites. If it can be called atomic
context, then item 1 applies here. If you're sure it can't, then
you should probably call might_sleep().

> +
> +     sg_init_one(&sg, req, sizeof(req));
> +     sgs[0] = &sg;
> +     sg_init_one(&ret, &req->ret, sizeof(req->ret));
> +     sgs[1] = &ret;
> +     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");
> +             spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> +             return -ENOSPC;
> +     }
> +     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->acked, req->done);
> +     err = req->ret;
> +     kfree(req);
> +
> +     return err;
> +};
> +
> +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/linux/virtio_pmem.h b/include/linux/virtio_pmem.h
> new file mode 100644
> index 0000000..0f83d9c
> --- /dev/null
> +++ b/include/linux/virtio_pmem.h
> @@ -0,0 +1,44 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * virtio_pmem.h: virtio pmem Driver
> + *
> + * Discovers persistent memory range information
> + * from host and provides a virtio based flushing
> + * interface.
> + */
> +#ifndef _LINUX_VIRTIO_PMEM_H
> +#define _LINUX_VIRTIO_PMEM_H
> +
> +#include <linux/virtio_ids.h>
> +#include <linux/virtio_config.h>
> +#include <uapi/linux/virtio_pmem.h>
> +#include <linux/libnvdimm.h>
> +#include <linux/spinlock.h>
> +
> +struct virtio_pmem_request {
> +     /* Host return status corresponding to flush request */
> +     int ret;
> +
> +     /* Wait queue to process deferred work after ack from host */
> +     wait_queue_head_t acked;
> +     bool done;
> +};
> +
> +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;
> +
> +     /* Synchronize virtqueue data */
> +     spinlock_t pmem_lock;
> +
> +     /* Memory region information */
> +     uint64_t start;
> +     uint64_t size;
> +};
> +#endif
> 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 <address@hidden>, 2018
> + */
> +#ifndef _UAPI_LINUX_VIRTIO_PMEM_H
> +#define _UAPI_LINUX_VIRTIO_PMEM_H
> +
> +struct virtio_pmem_config {
> +     __le64 start;
> +     __le64 size;
> +};
> +#endif