[4/4] vfio/pci: Allow MMIO regions to be exported through dma-buf

dma-buf has become a way to safely acquire a handle to non-struct page
memory that can still have lifetime controlled by the exporter. Notably
RDMA can now import dma-buf FDs and build them into MRs which allows for
PCI P2P operations. Extend this to allow vfio-pci to export MMIO memory
from PCI device BARs.

The patch design loosely follows the pattern in commit
db1a8dd916aa ("habanalabs: add support for dma-buf exporter") except this
does not support pinning.

Instead, this implements what, in the past, we've called a revocable
attachment using move. In normal situations the attachment is pinned, as a
BAR does not change physical address. However when the VFIO device is
closed, or a PCI reset is issued, access to the MMIO memory is revoked.

Revoked means that move occurs, but an attempt to immediately re-map the
memory will fail. In the reset case a future move will be triggered when
MMIO access returns. As both close and reset are under userspace control
it is expected that userspace will suspend use of the dma-buf before doing
these operations, the revoke is purely for kernel self-defense against a
hostile userspace.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
 drivers/vfio/pci/Makefile           |   1 +
 drivers/vfio/pci/vfio_pci_config.c  |   8 +-
 drivers/vfio/pci/vfio_pci_core.c    |  28 ++-
 drivers/vfio/pci/vfio_pci_dma_buf.c | 265 ++++++++++++++++++++++++++++
 drivers/vfio/pci/vfio_pci_priv.h    |  23 +++
 include/linux/vfio_pci_core.h       |   1 +
 include/uapi/linux/vfio.h           |  18 ++
 7 files changed, 336 insertions(+), 8 deletions(-)
 create mode 100644 drivers/vfio/pci/vfio_pci_dma_buf.c

On Sun, Aug 21, 2022 at 04:51:34PM +0300, Oded Gabbay wrote:

> > +static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,
> > +                                  struct sg_table *sgt,
> > +                                  enum dma_data_direction dir)
> > +{
> > +       struct vfio_pci_dma_buf *priv = attachment->dmabuf->priv;
> > +
> > +       dma_unmap_resource(attachment->dev, sg_dma_address(sgt->sgl),
> > +                          priv->dmabuf->size, dir, DMA_ATTR_SKIP_CPU_SYNC);
> > +       sg_free_table(sgt);
> Before calling sg_free_table(), you need to restore the orig_nents as
> it is used in that function to free the allocated memory of the sgt.

Oops, right, thanks good catch

Jason

On Wed, Aug 17, 2022 at 01:11:42PM -0300, Jason Gunthorpe wrote:
> dma-buf has become a way to safely acquire a handle to non-struct page
> memory that can still have lifetime controlled by the exporter. Notably
> RDMA can now import dma-buf FDs and build them into MRs which allows for
> PCI P2P operations. Extend this to allow vfio-pci to export MMIO memory
> from PCI device BARs.
> 
> The patch design loosely follows the pattern in commit
> db1a8dd916aa ("habanalabs: add support for dma-buf exporter") except this
> does not support pinning.
> 
> Instead, this implements what, in the past, we've called a revocable
> attachment using move. In normal situations the attachment is pinned, as a
> BAR does not change physical address. However when the VFIO device is
> closed, or a PCI reset is issued, access to the MMIO memory is revoked.
> 
> Revoked means that move occurs, but an attempt to immediately re-map the
> memory will fail. In the reset case a future move will be triggered when
> MMIO access returns. As both close and reset are under userspace control
> it is expected that userspace will suspend use of the dma-buf before doing
> these operations, the revoke is purely for kernel self-defense against a
> hostile userspace.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/vfio/pci/Makefile           |   1 +
>  drivers/vfio/pci/vfio_pci_config.c  |   8 +-
>  drivers/vfio/pci/vfio_pci_core.c    |  28 ++-
>  drivers/vfio/pci/vfio_pci_dma_buf.c | 265 ++++++++++++++++++++++++++++
>  drivers/vfio/pci/vfio_pci_priv.h    |  23 +++
>  include/linux/vfio_pci_core.h       |   1 +
>  include/uapi/linux/vfio.h           |  18 ++
>  7 files changed, 336 insertions(+), 8 deletions(-)
>  create mode 100644 drivers/vfio/pci/vfio_pci_dma_buf.c
> 
> diff --git a/drivers/vfio/pci/Makefile b/drivers/vfio/pci/Makefile
> index 24c524224da5a3..81006a157cde14 100644
> --- a/drivers/vfio/pci/Makefile
> +++ b/drivers/vfio/pci/Makefile
> @@ -2,6 +2,7 @@
>  
>  vfio-pci-core-y := vfio_pci_core.o vfio_pci_intrs.o vfio_pci_rdwr.o vfio_pci_config.o
>  vfio-pci-core-$(CONFIG_VFIO_PCI_ZDEV_KVM) += vfio_pci_zdev.o
> +vfio-pci-core-$(CONFIG_DMA_SHARED_BUFFER) += vfio_pci_dma_buf.o
>  obj-$(CONFIG_VFIO_PCI_CORE) += vfio-pci-core.o
>  
>  vfio-pci-y := vfio_pci.o
> diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
> index d22921efa25987..f8a9c24d04aeb7 100644
> --- a/drivers/vfio/pci/vfio_pci_config.c
> +++ b/drivers/vfio/pci/vfio_pci_config.c
> @@ -584,10 +584,12 @@ static int vfio_basic_config_write(struct vfio_pci_core_device *vdev, int pos,
>  		virt_mem = !!(le16_to_cpu(*virt_cmd) & PCI_COMMAND_MEMORY);
>  		new_mem = !!(new_cmd & PCI_COMMAND_MEMORY);
>  
> -		if (!new_mem)
> +		if (!new_mem) {
>  			vfio_pci_zap_and_down_write_memory_lock(vdev);
> -		else
> +			vfio_pci_dma_buf_move(vdev, true);
> +		} else {
>  			down_write(&vdev->memory_lock);
> +		}
>  
>  		/*
>  		 * If the user is writing mem/io enable (new_mem/io) and we
> @@ -622,6 +624,8 @@ static int vfio_basic_config_write(struct vfio_pci_core_device *vdev, int pos,
>  		*virt_cmd &= cpu_to_le16(~mask);
>  		*virt_cmd |= cpu_to_le16(new_cmd & mask);
>  
> +		if (__vfio_pci_memory_enabled(vdev))
> +			vfio_pci_dma_buf_move(vdev, false);
>  		up_write(&vdev->memory_lock);
>  	}
>  
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index d13e22021860cc..206f159c480e42 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -504,6 +504,8 @@ void vfio_pci_core_close_device(struct vfio_device *core_vdev)
>  	vfio_spapr_pci_eeh_release(vdev->pdev);
>  	vfio_pci_core_disable(vdev);
>  
> +	vfio_pci_dma_buf_cleanup(vdev);
> +
>  	mutex_lock(&vdev->igate);
>  	if (vdev->err_trigger) {
>  		eventfd_ctx_put(vdev->err_trigger);
> @@ -980,7 +982,10 @@ int vfio_pci_try_reset_function(struct vfio_pci_core_device *vdev)
>  	 */
>  	vfio_pci_set_power_state(vdev, PCI_D0);
>  
> +	vfio_pci_dma_buf_move(vdev, true);
>  	ret = pci_try_reset_function(vdev->pdev);
> +	if (__vfio_pci_memory_enabled(vdev))
> +		vfio_pci_dma_buf_move(vdev, false);
>  	up_write(&vdev->memory_lock);
>  
>  	return ret;
> @@ -1210,11 +1215,10 @@ long vfio_pci_core_ioctl(struct vfio_device *core_vdev, unsigned int cmd,
>  }
>  EXPORT_SYMBOL_GPL(vfio_pci_core_ioctl);
>  
> -static int vfio_pci_core_feature_token(struct vfio_device *device, u32 flags,
> -				       uuid_t __user *arg, size_t argsz)
> +static int vfio_pci_core_feature_token(struct vfio_pci_core_device *vdev,
> +				       u32 flags, uuid_t __user *arg,
> +				       size_t argsz)
>  {
> -	struct vfio_pci_core_device *vdev =
> -		container_of(device, struct vfio_pci_core_device, vdev);
>  	uuid_t uuid;
>  	int ret;
>  
> @@ -1241,9 +1245,14 @@ static int vfio_pci_core_feature_token(struct vfio_device *device, u32 flags,
>  int vfio_pci_core_ioctl_feature(struct vfio_device *device, u32 flags,
>  				void __user *arg, size_t argsz)
>  {
> +	struct vfio_pci_core_device *vdev =
> +		container_of(device, struct vfio_pci_core_device, vdev);
> +
>  	switch (flags & VFIO_DEVICE_FEATURE_MASK) {
>  	case VFIO_DEVICE_FEATURE_PCI_VF_TOKEN:
> -		return vfio_pci_core_feature_token(device, flags, arg, argsz);
> +		return vfio_pci_core_feature_token(vdev, flags, arg, argsz);
> +	case VFIO_DEVICE_FEATURE_DMA_BUF:
> +		return vfio_pci_core_feature_dma_buf(vdev, flags, arg, argsz);
>  	default:
>  		return -ENOTTY;
>  	}
> @@ -1881,6 +1890,7 @@ void vfio_pci_core_init_device(struct vfio_pci_core_device *vdev,
>  	INIT_LIST_HEAD(&vdev->vma_list);
>  	INIT_LIST_HEAD(&vdev->sriov_pfs_item);
>  	init_rwsem(&vdev->memory_lock);
> +	INIT_LIST_HEAD(&vdev->dmabufs);
>  }
>  EXPORT_SYMBOL_GPL(vfio_pci_core_init_device);
>  
> @@ -2227,11 +2237,17 @@ static int vfio_pci_dev_set_hot_reset(struct vfio_device_set *dev_set,
>  	 * cause the PCI config space reset without restoring the original
>  	 * state (saved locally in 'vdev->pm_save').
>  	 */
> -	list_for_each_entry(cur, &dev_set->device_list, vdev.dev_set_list)
> +	list_for_each_entry(cur, &dev_set->device_list, vdev.dev_set_list) {
> +		vfio_pci_dma_buf_move(cur, true);
>  		vfio_pci_set_power_state(cur, PCI_D0);
> +	}
>  
>  	ret = pci_reset_bus(pdev);
>  
> +	list_for_each_entry(cur, &dev_set->device_list, vdev.dev_set_list)
> +		if (__vfio_pci_memory_enabled(cur))
> +			vfio_pci_dma_buf_move(cur, false);
> +
>  err_undo:
>  	list_for_each_entry(cur, &dev_set->device_list, vdev.dev_set_list) {
>  		if (cur == cur_mem)
> diff --git a/drivers/vfio/pci/vfio_pci_dma_buf.c b/drivers/vfio/pci/vfio_pci_dma_buf.c
> new file mode 100644
> index 00000000000000..ac32405de5e48d
> --- /dev/null
> +++ b/drivers/vfio/pci/vfio_pci_dma_buf.c
> @@ -0,0 +1,265 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright (c) 2022, NVIDIA CORPORATION & AFFILIATES.
> + */
> +#include <linux/dma-buf.h>
> +#include <linux/pci-p2pdma.h>
> +#include <linux/dma-resv.h>
> +
> +#include "vfio_pci_priv.h"
> +
> +MODULE_IMPORT_NS(DMA_BUF);
> +
> +struct vfio_pci_dma_buf {
> +	struct dma_buf *dmabuf;
> +	struct vfio_pci_core_device *vdev;
> +	struct list_head dmabufs_elm;
> +	unsigned int index;
> +	size_t offset;
> +	bool revoked;
> +};
> +
> +static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
> +				   struct dma_buf_attachment *attachment)
> +{
> +	struct vfio_pci_dma_buf *priv = dmabuf->priv;
> +	int rc;
> +
> +	rc = pci_p2pdma_distance_many(priv->vdev->pdev, &attachment->dev, 1,
> +				      true);
> +	if (rc < 0)
> +		attachment->peer2peer = false;
> +	return 0;
> +}
> +
> +static void vfio_pci_dma_buf_unpin(struct dma_buf_attachment *attachment)
> +{
> +}
> +
> +static int vfio_pci_dma_buf_pin(struct dma_buf_attachment *attachment)
> +{
> +	/*
> +	 * Uses the dynamic interface but must always allow for
> +	 * dma_buf_move_notify() to do revoke
> +	 */
> +	return -EINVAL;
> +}
> +
> +static struct sg_table *
> +vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,
> +		     enum dma_data_direction dir)
> +{
> +	size_t sgl_size = dma_get_max_seg_size(attachment->dev);
> +	struct vfio_pci_dma_buf *priv = attachment->dmabuf->priv;
> +	struct scatterlist *sgl;
> +	struct sg_table *sgt;
> +	dma_addr_t dma_addr;
> +	unsigned int nents;
> +	size_t offset;
> +	int ret;
> +
> +	dma_resv_assert_held(priv->dmabuf->resv);
> +
> +	if (!attachment->peer2peer)
> +		return ERR_PTR(-EPERM);
> +
> +	if (!priv->revoked)
> +		return ERR_PTR(-ENODEV);
> +
> +	sgt = kzalloc(sizeof(*sgt), GFP_KERNEL);
> +	if (!sgt)
> +		return ERR_PTR(-ENOMEM);
> +
> +	nents = DIV_ROUND_UP(priv->dmabuf->size, sgl_size);
> +	ret = sg_alloc_table(sgt, nents, GFP_KERNEL);
> +	if (ret)
> +		goto err_kfree_sgt;
> +
> +	/*
> +	 * Since the memory being mapped is a device memory it could never be in
> +	 * CPU caches.
> +	 */
> +	dma_addr = dma_map_resource(
> +		attachment->dev,
> +		pci_resource_start(priv->vdev->pdev, priv->index) +
> +			priv->offset,
> +		priv->dmabuf->size, dir, DMA_ATTR_SKIP_CPU_SYNC);
dma_map_resource maps the phys to an IOVA in device's
default_domain, which, however, may not be the domain that the device is
currently attached to.
So, the importer of this sgt will convert this dma_addr back to phys?

BTW, I don't see the assignment of priv->index in below
vfio_pci_core_feature_dma_buf(), is it equal to get_dma_buf.region_index ?

Thanks
Yan

> +	ret = dma_mapping_error(attachment->dev, dma_addr);
> +	if (ret)
> +		goto err_free_sgt;
> +
> +	/*
> +	 * Break the BAR's physical range up into max sized SGL's according to
> +	 * the device's requirement.
> +	 */
> +	sgl = sgt->sgl;
> +	for (offset = 0; offset != priv->dmabuf->size;) {
> +		size_t chunk_size = min(priv->dmabuf->size - offset, sgl_size);
> +
> +		sg_set_page(sgl, NULL, chunk_size, 0);
> +		sg_dma_address(sgl) = dma_addr + offset;
> +		sg_dma_len(sgl) = chunk_size;
> +		sgl = sg_next(sgl);
> +		offset += chunk_size;
> +	}
> +
> +	/*
> +	 * Because we are not going to include a CPU list we want to have some
> +	 * chance that other users will detect this by setting the orig_nents to
> +	 * 0 and using only nents (length of DMA list) when going over the sgl
> +	 */
> +	sgt->orig_nents = 0;
> +	return sgt;
> +
> +err_free_sgt:
> +	sg_free_table(sgt);
> +err_kfree_sgt:
> +	kfree(sgt);
> +	return ERR_PTR(ret);
> +}
> +
> +static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,
> +				   struct sg_table *sgt,
> +				   enum dma_data_direction dir)
> +{
> +	struct vfio_pci_dma_buf *priv = attachment->dmabuf->priv;
> +
> +	dma_unmap_resource(attachment->dev, sg_dma_address(sgt->sgl),
> +			   priv->dmabuf->size, dir, DMA_ATTR_SKIP_CPU_SYNC);
> +	sg_free_table(sgt);
> +	kfree(sgt);
> +}
> +
> +static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf)
> +{
> +	struct vfio_pci_dma_buf *priv = dmabuf->priv;
> +
> +	/*
> +	 * Either this or vfio_pci_dma_buf_cleanup() will remove from the list.
> +	 * The refcount prevents both.
> +	 */
> +	if (priv->vdev) {
> +		down_write(&priv->vdev->memory_lock);
> +		list_del_init(&priv->dmabufs_elm);
> +		up_write(&priv->vdev->memory_lock);
> +		vfio_device_put(&priv->vdev->vdev);
> +	}
> +	kfree(priv);
> +}
> +
> +static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
> +	.attach = vfio_pci_dma_buf_attach,
> +	.map_dma_buf = vfio_pci_dma_buf_map,
> +	.pin = vfio_pci_dma_buf_pin,
> +	.unpin = vfio_pci_dma_buf_unpin,
> +	.release = vfio_pci_dma_buf_release,
> +	.unmap_dma_buf = vfio_pci_dma_buf_unmap,
> +};
> +
> +int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> +				  struct vfio_device_feature_dma_buf __user *arg,
> +				  size_t argsz)
> +{
> +	struct vfio_device_feature_dma_buf get_dma_buf;
> +	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
> +	struct vfio_pci_dma_buf *priv;
> +	int ret;
> +
> +	ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_GET,
> +				 sizeof(get_dma_buf));
> +	if (ret != 1)
> +		return ret;
> +
> +	if (copy_from_user(&get_dma_buf, arg, sizeof(get_dma_buf)))
> +		return -EFAULT;
> +
> +	/* For PCI the region_index is the BAR number like everything else */
> +	if (get_dma_buf.region_index >= VFIO_PCI_ROM_REGION_INDEX)
> +		return -EINVAL;
> +
> +	exp_info.ops = &vfio_pci_dmabuf_ops;
> +	exp_info.size = pci_resource_len(vdev->pdev, get_dma_buf.region_index);
> +	if (!exp_info.size)
> +		return -EINVAL;
> +	if (get_dma_buf.offset || get_dma_buf.length) {
> +		if (get_dma_buf.length > exp_info.size ||
> +		    get_dma_buf.offset >= exp_info.size ||
> +		    get_dma_buf.length > exp_info.size - get_dma_buf.offset ||
> +		    get_dma_buf.offset % PAGE_SIZE ||
> +		    get_dma_buf.length % PAGE_SIZE)
> +			return -EINVAL;
> +		exp_info.size = get_dma_buf.length;
> +	}
> +	exp_info.flags = get_dma_buf.open_flags;
> +
> +	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
> +	if (!priv)
> +		return -ENOMEM;
> +	INIT_LIST_HEAD(&priv->dmabufs_elm);
> +	priv->offset = get_dma_buf.offset;
> +
> +	exp_info.priv = priv;
> +	priv->dmabuf = dma_buf_export(&exp_info);
> +	if (IS_ERR(priv->dmabuf)) {
> +		ret = PTR_ERR(priv->dmabuf);
> +		kfree(priv);
> +		return ret;
> +	}
> +
> +	/* dma_buf_put() now frees priv */
> +
> +	down_write(&vdev->memory_lock);
> +	dma_resv_lock(priv->dmabuf->resv, NULL);
> +	priv->revoked = __vfio_pci_memory_enabled(vdev);
> +	priv->vdev = vdev;
> +	vfio_device_get(&vdev->vdev);
> +	list_add_tail(&priv->dmabufs_elm, &vdev->dmabufs);
> +	dma_resv_unlock(priv->dmabuf->resv);
> +	up_write(&vdev->memory_lock);
> +
> +	/*
> +	 * dma_buf_fd() consumes the reference, when the file closes the dmabuf
> +	 * will be released.
> +	 */
> +	return dma_buf_fd(priv->dmabuf, get_dma_buf.open_flags);
> +}
> +
> +void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
> +{
> +	struct vfio_pci_dma_buf *priv;
> +	struct vfio_pci_dma_buf *tmp;
> +
> +	lockdep_assert_held_write(&vdev->memory_lock);
> +
> +	list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) {
> +		if (!dma_buf_try_get(priv->dmabuf))
> +			continue;
> +		if (priv->revoked != revoked) {
> +			dma_resv_lock(priv->dmabuf->resv, NULL);
> +			priv->revoked = revoked;
> +			dma_buf_move_notify(priv->dmabuf);
> +			dma_resv_unlock(priv->dmabuf->resv);
> +		}
> +		dma_buf_put(priv->dmabuf);
> +	}
> +}
> +
> +void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
> +{
> +	struct vfio_pci_dma_buf *priv;
> +	struct vfio_pci_dma_buf *tmp;
> +
> +	down_write(&vdev->memory_lock);
> +	list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) {
> +		if (!dma_buf_try_get(priv->dmabuf))
> +			continue;
> +		dma_resv_lock(priv->dmabuf->resv, NULL);
> +		list_del_init(&priv->dmabufs_elm);
> +		priv->vdev = NULL;
> +		priv->revoked = true;
> +		dma_buf_move_notify(priv->dmabuf);
> +		dma_resv_unlock(priv->dmabuf->resv);
> +		vfio_device_put(&vdev->vdev);
> +		dma_buf_put(priv->dmabuf);
> +	}
> +	up_write(&vdev->memory_lock);
> +}
> diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
> index 5b1cb9a9838442..c295a1166e7a9c 100644
> --- a/drivers/vfio/pci/vfio_pci_priv.h
> +++ b/drivers/vfio/pci/vfio_pci_priv.h
> @@ -102,4 +102,27 @@ static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
>  	return (pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA;
>  }
>  
> +#ifdef CONFIG_DMA_SHARED_BUFFER
> +int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> +				  struct vfio_device_feature_dma_buf __user *arg,
> +				  size_t argsz);
> +void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev);
> +void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked);
> +#else
> +static int
> +vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
> +			      struct vfio_device_feature_dma_buf __user *arg,
> +			      size_t argsz)
> +{
> +	return -ENOTTY;
> +}
> +static inline void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
> +{
> +}
> +static inline void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev,
> +					 bool revoked)
> +{
> +}
> +#endif
> +
>  #endif
> diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h
> index 9d18b832e61a0d..b57f4ecc2665e1 100644
> --- a/include/linux/vfio_pci_core.h
> +++ b/include/linux/vfio_pci_core.h
> @@ -93,6 +93,7 @@ struct vfio_pci_core_device {
>  	struct mutex		vma_lock;
>  	struct list_head	vma_list;
>  	struct rw_semaphore	memory_lock;
> +	struct list_head	dmabufs;
>  };
>  
>  /* Will be exported for vfio pci drivers usage */
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index 733a1cddde30a5..1dcfad6dcb6863 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -986,6 +986,24 @@ enum vfio_device_mig_state {
>  	VFIO_DEVICE_STATE_RUNNING_P2P = 5,
>  };
>  
> +/**
> + * Upon VFIO_DEVICE_FEATURE_GET create a dma_buf fd for the
> + * region selected.
> + *
> + * open_flags are the typical flags passed to open(2), eg O_RDWR, O_CLOEXEC,
> + * etc. offset/length specify a slice of the region to create the dmabuf from.
> + * If both are 0 then the whole region is used.
> + *
> + * Return: The fd number on success, -1 and errno is set on failure.
> + */
> +struct vfio_device_feature_dma_buf {
> +	__u32 region_index;
> +	__u32 open_flags;
> +	__u32 offset;
> +	__u64 length;
> +};
> +#define VFIO_DEVICE_FEATURE_DMA_BUF 3
> +
>  /* -------- API for Type1 VFIO IOMMU -------- */
>  
>  /**
> -- 
> 2.37.2
>

On Mon, Aug 29, 2022 at 01:04:05PM +0800, Yan Zhao wrote:

> > +static struct sg_table *
> > +vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,
> > +		     enum dma_data_direction dir)
> > +{
> > +	size_t sgl_size = dma_get_max_seg_size(attachment->dev);
> > +	struct vfio_pci_dma_buf *priv = attachment->dmabuf->priv;
> > +	struct scatterlist *sgl;
> > +	struct sg_table *sgt;
> > +	dma_addr_t dma_addr;
> > +	unsigned int nents;
> > +	size_t offset;
> > +	int ret;
> > +
> > +	dma_resv_assert_held(priv->dmabuf->resv);
> > +
> > +	if (!attachment->peer2peer)
> > +		return ERR_PTR(-EPERM);
> > +
> > +	if (!priv->revoked)
> > +		return ERR_PTR(-ENODEV);
> > +
> > +	sgt = kzalloc(sizeof(*sgt), GFP_KERNEL);
> > +	if (!sgt)
> > +		return ERR_PTR(-ENOMEM);
> > +
> > +	nents = DIV_ROUND_UP(priv->dmabuf->size, sgl_size);
> > +	ret = sg_alloc_table(sgt, nents, GFP_KERNEL);
> > +	if (ret)
> > +		goto err_kfree_sgt;
> > +
> > +	/*
> > +	 * Since the memory being mapped is a device memory it could never be in
> > +	 * CPU caches.
> > +	 */
> > +	dma_addr = dma_map_resource(
> > +		attachment->dev,
> > +		pci_resource_start(priv->vdev->pdev, priv->index) +
> > +			priv->offset,
> > +		priv->dmabuf->size, dir, DMA_ATTR_SKIP_CPU_SYNC);
> dma_map_resource maps the phys to an IOVA in device's
> default_domain, which, however, may not be the domain that the device is
> currently attached to.

dmabuf is defined only in terms of the DMA API, so it creates a SGL
with DMA API mapped dma_addr's

> So, the importer of this sgt will convert this dma_addr back to phys?

No, it is illegal to call this API if the importer is not using the
DMA API.

I'm expecting to propose some new dmabuf API for this use-case, when
we get to it. For now all importers use the DMA API.

> BTW, I don't see the assignment of priv->index in below
> vfio_pci_core_feature_dma_buf(), is it equal to
> get_dma_buf.region_index ?

Yes, I noticed that too and have fixed it - the testing was all done
on index 0.

Jason