Re: [Qemu-devel] [PATCH v5 2/5] virtio-pmem: Add virtio pmem driver

[Yuval Shaia]

On Wed, Apr 10, 2019 at 02:24:26PM +0200, Cornelia Huck wrote:
> On Wed, 10 Apr 2019 09:38:22 +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/nvdimm/virtio_pmem.c     |  88 ++++++++++++++++++++++
> >  drivers/virtio/Kconfig           |  10 +++
> >  drivers/virtio/Makefile          |   1 +
> >  drivers/virtio/pmem.c            | 124 +++++++++++++++++++++++++++++++
> >  include/linux/virtio_pmem.h      |  60 +++++++++++++++
> >  include/uapi/linux/virtio_ids.h  |   1 +
> >  include/uapi/linux/virtio_pmem.h |  10 +++
> >  7 files changed, 294 insertions(+)
> >  create mode 100644 drivers/nvdimm/virtio_pmem.c
> >  create mode 100644 drivers/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/pmem.c b/drivers/virtio/pmem.c
> > new file mode 100644
> > index 000000000000..cc9de9589d56
> > --- /dev/null
> > +++ b/drivers/virtio/pmem.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 registers the virtual pmem device
> > + * with libnvdimm core.
> > + */
> > +#include <linux/virtio_pmem.h>
> > +#include <../../drivers/nvdimm/nd.h>
> > +
> > +static struct virtio_device_id id_table[] = {
> > +   { VIRTIO_ID_PMEM, VIRTIO_DEV_ANY_ID },
> > +   { 0 },
> > +};
> > +
> > + /* Initialize virt queue */
> > +static int init_vq(struct virtio_pmem *vpmem)
> 
> IMHO, you don't gain much by splitting off this function...

It make sense to have all the vq-init-related stuff in one function, so
here pmem_lock and req_list are used only for the vq.
Saying that - maybe it would be better to have the 3 in one struct.

> 
> > +{
> > +   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);
> 
> I'm personally not a fan of chained assignments... I think I'd just
> drop the 'vq' variable and operate on vpmem->req_vq directly.

+1

> 
> > +
> > +   spin_lock_init(&vpmem->pmem_lock);
> > +   INIT_LIST_HEAD(&vpmem->req_list);
> > +
> > +   return 0;
> > +};
> > +
> > +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",
> 
> Maybe s/config disabled/config access disabled/ ? That seems to be the
> more common message.
> 
> > +                   __func__);
> > +           return -EINVAL;
> > +   }
> > +
> > +   vdev->priv = vpmem = devm_kzalloc(&vdev->dev, sizeof(*vpmem),
> > +                                   GFP_KERNEL);
> 
> Here, the vpmem variable makes sense for convenience, but I'm again not
> a fan of the chaining :)

+1

> 
> > +   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);
> 
> And here :)
> 
> > +   if (!nvdimm_bus)
> > +           goto out_vq;
> > +
> > +   dev_set_drvdata(&vdev->dev, nvdimm_bus);
> > +
> > +   ndr_desc.res = &res;
> > +   ndr_desc.numa_node = nid;
> > +   ndr_desc.flush = virtio_pmem_flush;
> > +   set_bit(ND_REGION_PAGEMAP, &ndr_desc.flags);
> > +   set_bit(ND_REGION_ASYNC, &ndr_desc.flags);
> > +   nd_region = nvdimm_pmem_region_create(nvdimm_bus, &ndr_desc);
> > +   nd_region->provider_data =  dev_to_virtio
> > +                                   (nd_region->dev.parent->parent);
> 
> Isn't it clear that this parent chain will always end up at &vdev->dev?
> Maybe simply set ->provider_data to vdev directly? (Does it need to
> grab a reference count of the device, BTW?)
> 
> > +
> > +   if (!nd_region)
> > +           goto out_nd;
> 
> Probably better to do this check before you access nd_region's
> members :)
> 
> > +
> > +   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);
> 
> I haven't followed this around the nvdimm code, but is the nd_region
> you created during probe cleaned up automatically, or would you need to
> do something here?
> 
> > +   vdev->config->del_vqs(vdev);
> > +   vdev->config->reset(vdev);
> > +   kfree(vpmem);
> 
> You allocated vpmem via devm_kzalloc; isn't it freed automatically on
> remove?
> 
> > +}
> > +
> > +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,
> > +};
> > +
> > +module_virtio_driver(virtio_pmem_driver);
> > +MODULE_DEVICE_TABLE(virtio, id_table);
> > +MODULE_DESCRIPTION("Virtio pmem driver");
> > +MODULE_LICENSE("GPL");
> 
> Only looked at this from the general virtio driver angle; seems fine
> apart from some easy-to-fix issues and some personal style preference
> things.