Date:   Fri, 13 Oct 2017 10:44:31 +0100
From:   Stefan Hajnoczi <stefanha@...il.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,
        linux-mm@...ck.org, jack@...e.cz, stefanha@...hat.com,
        dan.j.williams@...el.com, riel@...hat.com,
        haozhong.zhang@...el.com, nilal@...hat.com, kwolf@...hat.com,
        pbonzini@...hat.com, ross.zwisler@...el.com, david@...hat.com,
        xiaoguangrong.eric@...il.com
Subject: Re: [RFC 2/2] KVM: add virtio-pmem driver

On Thu, Oct 12, 2017 at 09:20:26PM +0530, Pankaj Gupta wrote:
>   This patch adds virtio-pmem driver for KVM guest.
>   Guest reads the persistent memory range information
>   over virtio bus from Qemu and reserves the range
>   as persistent memory. Guest also allocates a block
>   device corresponding to the pmem range which later
>   can be accessed with DAX compatible file systems.
>   Idea is to use the virtio channel between guest and
>   host to perform the block device flush for guest pmem 
>   DAX device.
> 
>   There is work to do including DAX file system support 
>   and other advanced features.
> 
> Signed-off-by: Pankaj Gupta <pagupta@...hat.com>
> ---
>  drivers/virtio/Kconfig           |  10 ++
>  drivers/virtio/Makefile          |   1 +
>  drivers/virtio/virtio_pmem.c     | 322 +++++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/virtio_pmem.h |  55 +++++++
>  4 files changed, 388 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 cff773f15b7e..0192c4bda54b 100644
> --- a/drivers/virtio/Kconfig
> +++ b/drivers/virtio/Kconfig
> @@ -38,6 +38,16 @@ config VIRTIO_PCI_LEGACY
>  
>  	  If unsure, say Y.
>  
> +config VIRTIO_PMEM
> +	tristate "Virtio pmem driver"
> +	depends on VIRTIO
> +	---help---
> +	 This driver adds persistent memory range within a KVM guest.
> +         It also associates a block device corresponding to the pmem
> +	 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 41e30e3dc842..032ade725cc2 100644
> --- a/drivers/virtio/Makefile
> +++ b/drivers/virtio/Makefile
> @@ -5,3 +5,4 @@ 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 000000000000..74e47cae0e24
> --- /dev/null
> +++ b/drivers/virtio/virtio_pmem.c
> @@ -0,0 +1,322 @@
> +/*
> + * virtio-pmem driver
> + */
> +
> +#include <linux/virtio.h>
> +#include <linux/swap.h>
> +#include <linux/workqueue.h>
> +#include <linux/delay.h>
> +#include <linux/slab.h>
> +#include <linux/module.h>
> +#include <linux/oom.h>
> +#include <linux/wait.h>
> +#include <linux/mm.h>
> +#include <linux/mount.h>
> +#include <linux/magic.h>
> +#include <linux/virtio_pmem.h>
> +
> +void devm_vpmem_disable(struct device *dev, struct resource *res, void *addr)
> +{
> +	devm_memunmap(dev, addr);
> +	devm_release_mem_region(dev, res->start, resource_size(res));
> +}
> +
> +static void pmem_flush_done(struct virtqueue *vq)
> +{
> +	return;
> +};
> +
> +static void virtio_pmem_release_queue(void *q)
> +{
> +	blk_cleanup_queue(q);
> +}
> +
> +static void virtio_pmem_freeze_queue(void *q)
> +{
> +	blk_freeze_queue_start(q);
> +}
> +
> +static void virtio_pmem_release_disk(void *__pmem)
> +{
> +	struct virtio_pmem *pmem = __pmem;
> +
> +	del_gendisk(pmem->disk);
> +	put_disk(pmem->disk);
> +}
> +
> +static int init_vq(struct virtio_pmem *vpmem)
> +{
> +	struct virtqueue *vq;
> +
> +	/* single vq */
> +	vq = virtio_find_single_vq(vpmem->vdev, pmem_flush_done, "flush_queue");
> +
> +	if (IS_ERR(vq))
> +		return PTR_ERR(vq);
> +
> +	return 0;
> +}
> +
> +static struct vmem_altmap *setup_pmem_pfn(struct virtio_pmem *vpmem,
> +			struct resource *res, struct vmem_altmap *altmap)
> +{
> +	u32 start_pad = 0, end_trunc = 0;
> +	resource_size_t start, size;
> +	unsigned long npfns;
> +	phys_addr_t offset;
> +
> +	size = resource_size(res);
> +	start = PHYS_SECTION_ALIGN_DOWN(res->start);
> +
> +	if (region_intersects(start, size, IORESOURCE_SYSTEM_RAM,
> +		IORES_DESC_NONE) == REGION_MIXED) {
> +
> +		start = res->start;
> +		start_pad = PHYS_SECTION_ALIGN_UP(start) - start;
> +	}
> +	start = res->start;
> +	size = PHYS_SECTION_ALIGN_UP(start + size) - start;
> +
> +	if (region_intersects(start, size, IORESOURCE_SYSTEM_RAM,
> +		IORES_DESC_NONE) == REGION_MIXED) {
> +
> +		size = resource_size(res);
> +		end_trunc = start + size -
> +				PHYS_SECTION_ALIGN_DOWN(start + size);
> +	}
> +
> +	start += start_pad;
> +	size = resource_size(res);
> +	npfns = PFN_SECTION_ALIGN_UP((size - start_pad - end_trunc - SZ_8K)
> +						/ PAGE_SIZE);
> +
> +      /*
> +       * vmemmap_populate_hugepages() allocates the memmap array in
> +       * HPAGE_SIZE chunks.
> +       */
> +	offset = ALIGN(start + SZ_8K + 64 * npfns, HPAGE_SIZE) - start;
> +	vpmem->data_offset = offset;
> +
> +	struct vmem_altmap __altmap = {
> +		.base_pfn = init_altmap_base(start+start_pad),
> +		.reserve = init_altmap_reserve(start+start_pad),
> +	};
> +
> +	res->start += start_pad;
> +	res->end -= end_trunc;
> +	memcpy(altmap, &__altmap, sizeof(*altmap));
> +	altmap->free = PHYS_PFN(offset - SZ_8K);
> +	altmap->alloc = 0;

The __altmap part can be rewritten using compound literal syntax:

  *altmap = (struct vmem_altmap) {
      .base_pfn = init_altmap_base(start+start_pad),
      .reserve = init_altmap_reserve(start+start_pad),
      .free = PHYS_PFN(offset - SZ_8K),
  };

This eliminates the temporary variable, memcpy, and explicit alloc = 0
initialization.

> +
> +	return altmap;
> +}
> +
> +static blk_status_t pmem_do_bvec(struct virtio_pmem *pmem, struct page *page,
> +			unsigned int len, unsigned int off, bool is_write,
> +			sector_t sector)
> +{
> +	blk_status_t rc = BLK_STS_OK;
> +	phys_addr_t pmem_off = sector * 512 + pmem->data_offset;
> +	void *pmem_addr = pmem->virt_addr + pmem_off;
> +
> +	if (!is_write) {
> +		rc = read_pmem(page, off, pmem_addr, len);
> +			flush_dcache_page(page);
> +	} else {
> +		flush_dcache_page(page);

What are the semantics of this device?  Why flush the dcache here if an
explicit flush command needs to be sent over the virtqueue?

> +		write_pmem(pmem_addr, page, off, len);
> +	}
> +
> +	return rc;
> +}
> +
> +static int vpmem_rw_page(struct block_device *bdev, sector_t sector,
> +		       struct page *page, bool is_write)
> +{
> +	struct virtio_pmem  *pmem = bdev->bd_queue->queuedata;
> +	blk_status_t rc;
> +
> +	rc = pmem_do_bvec(pmem, page, hpage_nr_pages(page) * PAGE_SIZE,
> +			  0, is_write, sector);
> +
> +	if (rc == 0)
> +		page_endio(page, is_write, 0);
> +
> +	return blk_status_to_errno(rc);
> +}
> +
> +#ifndef REQ_FLUSH
> +#define REQ_FLUSH REQ_PREFLUSH
> +#endif

Is this out-of-tree kernel module compatibility stuff that can be
removed?

> +
> +static blk_qc_t virtio_pmem_make_request(struct request_queue *q,
> +			struct bio *bio)
> +{
> +	blk_status_t rc = 0;
> +	struct bio_vec bvec;
> +	struct bvec_iter iter;
> +	struct virtio_pmem *pmem = q->queuedata;
> +
> +	if (bio->bi_opf & REQ_FLUSH)
> +		//todo host flush command

This detail is critical to the device design.  What is the plan?

> +
> +	bio_for_each_segment(bvec, bio, iter) {
> +		rc = pmem_do_bvec(pmem, bvec.bv_page, bvec.bv_len,
> +				bvec.bv_offset, op_is_write(bio_op(bio)),
> +				iter.bi_sector);
> +		if (rc) {
> +			bio->bi_status = rc;
> +			break;
> +		}
> +	}
> +
> +	bio_endio(bio);
> +	return BLK_QC_T_NONE;
> +}
> +
> +static const struct block_device_operations pmem_fops = {
> +	.owner =		THIS_MODULE,
> +	.rw_page =		vpmem_rw_page,
> +	//.revalidate_disk =	nvdimm_revalidate_disk,
> +};
> +
> +static int virtio_pmem_probe(struct virtio_device *vdev)
> +{
> +	struct virtio_pmem *vpmem;
> +	int err = 0;
> +	void *addr;
> +	struct resource *res, res_pfn;
> +	struct request_queue *q;
> +	struct vmem_altmap __altmap, *altmap = NULL;
> +	struct gendisk *disk;
> +	struct device *gendev;
> +	int nid = dev_to_node(&vdev->dev);
> +
> +	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;
> +	}
> +
> +	dev_set_drvdata(&vdev->dev, vpmem);
> +
> +	vpmem->vdev = vdev;
> +	err = init_vq(vpmem);
> +	if (err)
> +		goto out;
> +
> +	if (!virtio_has_feature(vdev, VIRTIO_PMEM_PLUG)) {
> +		dev_err(&vdev->dev, "%s : pmem not supported\n",
> +			__func__);
> +		goto out;
> +	}

Why is VIRTIO_PMEM_PLUG optional for this new device type if it's
already required by the first version of the driver?

err is not set when the error path is taken.

> +
> +	virtio_cread(vpmem->vdev, struct virtio_pmem_config,
> +			start, &vpmem->start);
> +	virtio_cread(vpmem->vdev, struct virtio_pmem_config,
> +			size, &vpmem->size);

The struct resource start and size fields can vary between
architectures.  Virtio device configuration space layout must not vary
between architectures.  Please use u64.

> +
> +	res_pfn.start = vpmem->start;
> +	res_pfn.end   = vpmem->start + vpmem->size-1;
> +
> +	/* used for allocating memmap in the pmem device */
> +	altmap	      = setup_pmem_pfn(vpmem, &res_pfn, &__altmap);
> +
> +	res = devm_request_mem_region(&vdev->dev,
> +			res_pfn.start, resource_size(&res_pfn), "virtio-pmem");
> +
> +	if (!res) {
> +		dev_warn(&vdev->dev, "could not reserve region ");
> +		return -EBUSY;
> +	}
> +
> +	q = blk_alloc_queue_node(GFP_KERNEL, dev_to_node(&vdev->dev));
> +
> +	if (!q)
> +		return -ENOMEM;
> +
> +	if (devm_add_action_or_reset(&vdev->dev,
> +				virtio_pmem_release_queue, q))
> +		return -ENOMEM;
> +
> +	vpmem->pfn_flags = PFN_DEV;
> +
> +	/* allocate memap in pmem device itself */
> +	if (IS_ENABLED(CONFIG_ZONE_DEVICE)) {
> +
> +		addr = devm_memremap_pages(&vdev->dev, res,
> +				&q->q_usage_counter, altmap);
> +		vpmem->pfn_flags |= PFN_MAP;
> +	} else
> +		addr = devm_memremap(&vdev->dev, vpmem->start,
> +				vpmem->size, ARCH_MEMREMAP_PMEM);
> +
> +        /*
> +         * At release time the queue must be frozen before
> +         * devm_memremap_pages is unwound
> +         */
> +	if (devm_add_action_or_reset(&vdev->dev,
> +				virtio_pmem_freeze_queue, q))
> +		return -ENOMEM;
> +
> +	if (IS_ERR(addr))
> +		return PTR_ERR(addr);
> +
> +	vpmem->virt_addr = addr;
> +	blk_queue_write_cache(q, 0, 0);
> +	blk_queue_make_request(q, virtio_pmem_make_request);
> +	blk_queue_physical_block_size(q, PAGE_SIZE);
> +	blk_queue_logical_block_size(q, 512);
> +	blk_queue_max_hw_sectors(q, UINT_MAX);
> +	queue_flag_set_unlocked(QUEUE_FLAG_NONROT, q);
> +	queue_flag_set_unlocked(QUEUE_FLAG_DAX, q);
> +	q->queuedata = vpmem;
> +
> +	disk = alloc_disk_node(0, nid);
> +	if (!disk)
> +		return -ENOMEM;

The return statements in this function look suspicious.  There probably
needs to be a cleanup to avoid memory leaks.

> +	vpmem->disk = disk;
> +
> +	disk->fops                = &pmem_fops;
> +	disk->queue               = q;
> +	disk->flags               = GENHD_FL_EXT_DEVT;
> +	strcpy(disk->disk_name, "vpmem");
> +	set_capacity(disk, vpmem->size/512);
> +	gendev = disk_to_dev(disk);
> +
> +	virtio_device_ready(vdev);
> +	device_add_disk(&vdev->dev, disk);
> +
> +	if (devm_add_action_or_reset(&vdev->dev,
> +			virtio_pmem_release_disk, vpmem))
> +		return -ENOMEM;
> +
> +	revalidate_disk(disk);
> +	return 0;
> +out:
> +	vdev->config->del_vqs(vdev);
> +	return err;
> +}
> +
> +static struct virtio_driver virtio_pmem_driver = {
> +	.feature_table		= features,
> +	.feature_table_size	= ARRAY_SIZE(features),
> +	.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");
> diff --git a/include/uapi/linux/virtio_pmem.h b/include/uapi/linux/virtio_pmem.h
> new file mode 100644
> index 000000000000..ec0c728c79ba
> --- /dev/null
> +++ b/include/uapi/linux/virtio_pmem.h
> @@ -0,0 +1,55 @@
> +/*
> + * Virtio pmem Device
> + *
> + *
> + */
> +
> +#ifndef _LINUX_VIRTIO_PMEM_H
> +#define _LINUX_VIRTIO_PMEM_H
> +
> +#include <linux/types.h>
> +#include <linux/virtio_types.h>
> +#include <linux/virtio_ids.h>
> +#include <linux/virtio_config.h>
> +#include <linux/pfn_t.h>
> +#include <linux/fs.h>
> +#include <linux/blk-mq.h>
> +#include <linux/pmem_common.h>

include/uapi/ headers must compile when included from userspace
applications.  The header should define userspace APIs only.

If you want to define kernel-internal APIs, please add them to
include/linux/ or similar.

This also means that include/uapi/ headers cannot include
include/linux/pmem_common.h or other kernel headers outside #ifdef
__KERNEL__.

This header file should only contain struct virtio_pmem_config,
VIRTIO_PMEM_PLUG, and other virtio device definitions.

> +
> +bool pmem_should_map_pages(struct device *dev);
> +
> +#define VIRTIO_PMEM_PLUG 0
> +
> +struct virtio_pmem_config {
> +
> +uint64_t start;
> +uint64_t size;
> +uint64_t align;
> +uint64_t data_offset;
> +};
> +
> +struct virtio_pmem {
> +
> +	struct virtio_device *vdev;
> +	struct virtqueue *req_vq;
> +
> +	uint64_t start;
> +	uint64_t size;
> +	uint64_t align;
> +	uint64_t data_offset;
> +	u64 pfn_flags;
> +	void *virt_addr;
> +	struct gendisk *disk;
> +} __packed;
> +
> +static struct virtio_device_id id_table[] = {
> +	{ VIRTIO_ID_PMEM, VIRTIO_DEV_ANY_ID },
> +	{ 0 },
> +};
> +
> +static unsigned int features[] = {
> +	VIRTIO_PMEM_PLUG,
> +};
> +
> +#endif
> +
> -- 
> 2.13.0
> 

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