Re: [PATCH v8 21/24] vfio: Add VFIO_DEVICE_BIND_IOMMUFD

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> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Thursday, March 30, 2023 5:01 AM
> 
> On Mon, 27 Mar 2023 02:40:44 -0700
> Yi Liu <yi.l.liu@xxxxxxxxx> wrote:
> 
> > This adds ioctl for userspace to bind device cdev fd to iommufd.
> >
> >     VFIO_DEVICE_BIND_IOMMUFD: bind device to an iommufd, hence gain DMA
> > 			      control provided by the iommufd. open_device
> > 			      op is called after bind_iommufd op.
> > 			      VFIO no iommu mode is indicated by passing
> > 			      a negative iommufd value.
> >
> > Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> > Tested-by: Terrence Xu <terrence.xu@xxxxxxxxx>
> > Signed-off-by: Yi Liu <yi.l.liu@xxxxxxxxx>
> > ---
> >  drivers/vfio/device_cdev.c | 153 +++++++++++++++++++++++++++++++++++++
> >  drivers/vfio/vfio.h        |  13 ++++
> >  drivers/vfio/vfio_main.c   |   5 ++
> >  include/uapi/linux/vfio.h  |  37 +++++++++
> >  4 files changed, 208 insertions(+)
> >
> > diff --git a/drivers/vfio/device_cdev.c b/drivers/vfio/device_cdev.c
> > index 1c640016a824..2b563bac50b9 100644
> > --- a/drivers/vfio/device_cdev.c
> > +++ b/drivers/vfio/device_cdev.c
> > @@ -3,6 +3,7 @@
> >   * Copyright (c) 2023 Intel Corporation.
> >   */
> >  #include <linux/vfio.h>
> > +#include <linux/iommufd.h>
> >
> >  #include "vfio.h"
> >
> > @@ -44,6 +45,158 @@ int vfio_device_fops_cdev_open(struct inode *inode, struct
> file *filep)
> >  	return ret;
> >  }
> >
> > +static void vfio_device_get_kvm_safe(struct vfio_device_file *df)
> > +{
> > +	spin_lock(&df->kvm_ref_lock);
> > +	if (df->kvm)
> > +		_vfio_device_get_kvm_safe(df->device, df->kvm);
> > +	spin_unlock(&df->kvm_ref_lock);
> > +}
> > +
> > +void vfio_device_cdev_close(struct vfio_device_file *df)
> > +{
> > +	struct vfio_device *device = df->device;
> > +
> > +	/*
> > +	 * As df->access_granted writer is under dev_set->lock as well,
> > +	 * so this read no need to use smp_load_acquire() to pair with
> 
> Nit, "no need to use" -> "does not need to use"

got it.

> 
> > +	 * smp_store_release() in the caller of vfio_device_open().
> > +	 */
> > +	if (!df->access_granted)
> > +		return;
> > +
> 
> Isn't the lock we're acquiring below the one that we claim to have in
> the comment above to make the non-smp_load_acquire() test safe?

the comment may be not accurate enough. The the non-smp_load_acquire()
and no lock test were according to the below two remarks in v4 and v5.

https://lore.kernel.org/kvm/Y%2FYRx7jLuyEoLxZg@xxxxxxxxxx/
https://lore.kernel.org/kvm/Y%2F0CV1K0YNHA+olf@xxxxxxxxxx/

Perhaps the comment should be:

"In the time of close, there is no contention with another one
  changing this flag. So test df->access_granted without lock
  nor smp_load_acquire() is ok."

> > +	mutex_lock(&device->dev_set->lock);
> > +	vfio_device_close(df);
> > +	vfio_device_put_kvm(device);
> > +	if (df->iommufd)
> > +		iommufd_ctx_put(df->iommufd);
> > +	mutex_unlock(&device->dev_set->lock);
> > +	vfio_device_unblock_group(device);
> > +}
> > +
> > +static int vfio_device_cdev_enable_noiommu(struct vfio_device *device)
> > +{
> > +	if (!capable(CAP_SYS_RAWIO))
> > +		return -EPERM;
> > +
> > +	if (!device->noiommu)
> > +		return -EINVAL;
> > +
> > +	return 0;
> > +}
> 
> This is testing, not enabling. ie. naming nit.

how about probe_noiommu or test_noiommu?

> 
> > +
> > +static struct iommufd_ctx *vfio_get_iommufd_from_fd(int fd)
> > +{
> > +	struct fd f;
> > +	struct iommufd_ctx *iommufd;
> > +
> > +	f = fdget(fd);
> > +	if (!f.file)
> > +		return ERR_PTR(-EBADF);
> > +
> > +	iommufd = iommufd_ctx_from_file(f.file);
> > +
> > +	fdput(f);
> > +	return iommufd;
> > +}
> > +
> > +long vfio_device_ioctl_bind_iommufd(struct vfio_device_file *df,
> > +				    struct vfio_device_bind_iommufd __user *arg)
> > +{
> > +	struct vfio_device *device = df->device;
> > +	struct vfio_device_bind_iommufd bind;
> > +	struct iommufd_ctx *iommufd = NULL;
> > +	unsigned long minsz;
> > +	int ret;
> > +
> > +	static_assert(__same_type(arg->out_devid, bind.out_devid));
> 
> They're the same field in the same structure, how could they be
> otherwise?

@Jason, should I remove this check?

> > +
> > +	minsz = offsetofend(struct vfio_device_bind_iommufd, out_devid);
> > +
> > +	if (copy_from_user(&bind, arg, minsz))
> > +		return -EFAULT;
> > +
> > +	if (bind.argsz < minsz || bind.flags)
> > +		return -EINVAL;
> > +
> > +	if (!device->ops->bind_iommufd)
> > +		return -ENODEV;
> 
> This test seems beyond normal paranoia since we test in
> __vfio_register_dev()

yes. The whole c file depends on VFIO_DEVICE_CDEV which
depends on IOMMUFD, and if IOMMUFD is enabled,
__vfio_register_dev() already checks this callback.

> 
> > +
> > +	/* BIND_IOMMUFD only allowed for cdev fds */
> > +	if (df->group)
> > +		return -EINVAL;
> > +
> > +	ret = vfio_device_block_group(device);
> > +	if (ret)
> > +		return ret;
> > +
> > +	mutex_lock(&device->dev_set->lock);
> > +	/* one device cannot be bound twice */
> > +	if (df->access_granted) {
> > +		ret = -EINVAL;
> > +		goto out_unlock;
> > +	}
> > +
> > +	/* iommufd < 0 means noiommu mode */
> > +	if (bind.iommufd < 0) {
> > +		ret = vfio_device_cdev_enable_noiommu(device);
> > +		if (ret)
> > +			goto out_unlock;
> > +	} else {
> > +		iommufd = vfio_get_iommufd_from_fd(bind.iommufd);
> > +		if (IS_ERR(iommufd)) {
> > +			ret = PTR_ERR(iommufd);
> > +			goto out_unlock;
> > +		}
> > +	}
> > +
> > +	/*
> > +	 * Before the device open, get the KVM pointer currently
> > +	 * associated with the device file (if there is) and obtain
> > +	 * a reference.  This reference is held until device closed.
> > +	 * Save the pointer in the device for use by drivers.
> > +	 */
> > +	vfio_device_get_kvm_safe(df);
> > +
> > +	df->iommufd = iommufd;
> > +	ret = vfio_device_open(df);
> > +	if (ret)
> > +		goto out_put_kvm;
> > +
> > +	if (df->iommufd)
> > +		bind.out_devid = df->devid;
> 
> How about only setting df->iommufd in the non-noiommu case above so
> it's not confusing that it was just set 4 lines previous.  That also
> allows the iommufd pointer to be scoped within that branch and not
> require initialization.  It might make sense to declare:
> 
> 	bool is_noiommu = (bind.iommufd < 0);
> 
> and use it consistently rather than alternating testing between
> bind.iommufd and df->iommufd.

sure.

> > +
> > +	ret = copy_to_user(&arg->out_devid, &bind.out_devid,
> > +			   sizeof(bind.out_devid)) ? -EFAULT : 0;
> 
> In the noiommu case, this copies back the input value, shouldn't it be
> some known invalid value?  Seems confusing.

maybe just do copy for the non-noiommu case?

> > +	if (ret)
> > +		goto out_close_device;
> > +
> > +	if (bind.iommufd < 0)
> > +		dev_warn(device->dev, "device is bound to vfio-noiommu by user "
> > +			 "(%s:%d)\n", current->comm, task_pid_nr(current));
> > +
> > +	/*
> > +	 * Paired with smp_load_acquire() in vfio_device_fops::ioctl/
> > +	 * read/write/mmap
> > +	 */
> > +	smp_store_release(&df->access_granted, true);
> > +	mutex_unlock(&device->dev_set->lock);
> > +
> > +	return 0;
> > +
> > +out_close_device:
> > +	vfio_device_close(df);
> > +out_put_kvm:
> > +	df->iommufd = NULL;
> > +	vfio_device_put_kvm(device);
> > +	if (iommufd)
> > +		iommufd_ctx_put(iommufd);
> > +out_unlock:
> > +	mutex_unlock(&device->dev_set->lock);
> > +	vfio_device_unblock_group(device);
> > +	return ret;
> > +}
> > +
> >  static char *vfio_device_devnode(const struct device *dev, umode_t *mode)
> >  {
> >  	return kasprintf(GFP_KERNEL, "vfio/devices/%s", dev_name(dev));
> > diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> > index 3a8fd0e32f59..ace3d52b0928 100644
> > --- a/drivers/vfio/vfio.h
> > +++ b/drivers/vfio/vfio.h
> > @@ -281,6 +281,9 @@ static inline void vfio_device_del(struct vfio_device *device)
> >
> >  void vfio_init_device_cdev(struct vfio_device *device);
> >  int vfio_device_fops_cdev_open(struct inode *inode, struct file *filep);
> > +void vfio_device_cdev_close(struct vfio_device_file *df);
> > +long vfio_device_ioctl_bind_iommufd(struct vfio_device_file *df,
> > +				    struct vfio_device_bind_iommufd __user *arg);
> >  int vfio_cdev_init(struct class *device_class);
> >  void vfio_cdev_cleanup(void);
> >  #else
> > @@ -304,6 +307,16 @@ static inline int vfio_device_fops_cdev_open(struct inode
> *inode,
> >  	return 0;
> >  }
> >
> > +static inline void vfio_device_cdev_close(struct vfio_device_file *df)
> > +{
> > +}
> > +
> > +static inline long vfio_device_ioctl_bind_iommufd(struct vfio_device_file *df,
> > +						  struct vfio_device_bind_iommufd
> __user *arg)
> > +{
> > +	return -EOPNOTSUPP;
> > +}
> > +
> >  static inline int vfio_cdev_init(struct class *device_class)
> >  {
> >  	return 0;
> > diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> > index 58fc3bb768f2..375086c8803f 100644
> > --- a/drivers/vfio/vfio_main.c
> > +++ b/drivers/vfio/vfio_main.c
> > @@ -559,6 +559,8 @@ static int vfio_device_fops_release(struct inode *inode,
> struct file *filep)
> >
> >  	if (df->group)
> >  		vfio_device_group_close(df);
> > +	else
> > +		vfio_device_cdev_close(df);
> >
> >  	vfio_device_put_registration(device);
> >
> > @@ -1132,6 +1134,9 @@ static long vfio_device_fops_unl_ioctl(struct file *filep,
> >  	struct vfio_device *device = df->device;
> >  	int ret;
> >
> > +	if (cmd == VFIO_DEVICE_BIND_IOMMUFD)
> > +		return vfio_device_ioctl_bind_iommufd(df, (void __user *)arg);
> > +
> >  	/* Paired with smp_store_release() following vfio_device_open() */
> >  	if (!smp_load_acquire(&df->access_granted))
> >  		return -EINVAL;
> > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > index 61b801dfd40b..62b2f2497525 100644
> > --- a/include/uapi/linux/vfio.h
> > +++ b/include/uapi/linux/vfio.h
> > @@ -194,6 +194,43 @@ struct vfio_group_status {
> >
> >  /* --------------- IOCTLs for DEVICE file descriptors --------------- */
> >
> > +/*
> > + * VFIO_DEVICE_BIND_IOMMUFD - _IOR(VFIO_TYPE, VFIO_BASE + 19,
> > + *				   struct vfio_device_bind_iommufd)
> > + *
> > + * Bind a vfio_device to the specified iommufd.
> > + *
> > + * The user should provide a device cookie when calling this ioctl. The
> > + * cookie is carried only in event e.g. I/O fault reported to userspace
> > + * via iommufd. The user should use devid returned by this ioctl to mark
> > + * the target device in other ioctls (e.g. iommu hardware infomration query
> > + * via iommufd, and etc.).
> 
> AFAICT, the whole concept of this dev_cookie is a fantasy.  It only
> exists in this series in these comments and the structure below.  It's
> not even defined whether it needs to be unique within an iommufd
> context, and clearly nothing here validates that.  There's not enough
> implementation for it to exist in this series.  Maybe dev_cookie is
> appended to the end of the structure and indicated with a flag when it
> has some meaning.

sorry, I should have deleted it. ☹

> 
> > + *
> > + * User is not allowed to access the device before the binding operation
> > + * is completed.
> 
> s/not allowed to access/restricted from accessing/

got it.

> 
> > + *
> > + * Unbind is automatically conducted when device fd is closed.
> > + *
> > + * @argsz:	 user filled size of this data.
> > + * @flags:	 reserved for future extension.
> > + * @dev_cookie:	 a per device cookie provided by userspace.
> > + * @iommufd:	 iommufd to bind. a negative value means noiommu.
> 
> "Use a negative value for no-iommu, where supported", or better, should
> we define this explicitly as -1, or why not use a flag bit to specify
> no-iommu?  Maybe minsz is only through flags for the noiommu use case.
> Thanks,

I don’t have preference here. maybe using -1 can save a flag bit for future
extension. 

> 
> > + * @out_devid:	 the device id generated by this bind. This field is valid
> > + *		as long as the input @iommufd is valid. Otherwise, it is
> > + *		meaningless.
> > + *
> > + * Return: 0 on success, -errno on failure.
> > + */
> > +struct vfio_device_bind_iommufd {
> > +	__u32		argsz;
> > +	__u32		flags;
> > +	__aligned_u64	dev_cookie;
> > +	__s32		iommufd;
> > +	__u32		out_devid;
> > +};
> > +
> > +#define VFIO_DEVICE_BIND_IOMMUFD	_IO(VFIO_TYPE, VFIO_BASE + 19)
> > +
> >  /**
> >   * VFIO_DEVICE_GET_INFO - _IOR(VFIO_TYPE, VFIO_BASE + 7,
> >   *						struct vfio_device_info)

Regards,
Yi Liu




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