RE: [PATCH v12 07/24] vfio: Block device access via device fd until device is opened

Liu, Yi L
2023-06-13T05:46:45Z

> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Tuesday, June 13, 2023 5:52 AM
>
> On Fri, 2 Jun 2023 05:16:36 -0700
> Yi Liu <yi.l.liu@intel.com> wrote:
>
> > Allow the vfio_device file to be in a state where the device FD is
> > opened but the device cannot be used by userspace (i.e. its .open_device()
> > hasn't been called). This inbetween state is not used when the device
> > FD is spawned from the group FD, however when we create the device FD
> > directly by opening a cdev it will be opened in the blocked state.
> >
> > The reason for the inbetween state is that userspace only gets a FD but
> > doesn't gain access permission until binding the FD to an iommufd. So in
> > the blocked state, only the bind operation is allowed. Completing bind
> > will allow user to further access the device.
> >
> > This is implemented by adding a flag in struct vfio_device_file to mark
> > the blocked state and using a simple smp_load_acquire() to obtain the
> > flag value and serialize all the device setup with the thread accessing
> > this device.
> >
> > Following this lockless scheme, it can safely handle the device FD
> > unbound->bound but it cannot handle bound->unbound. To allow this we'd
> > need to add a lock on all the vfio ioctls which seems costly. So once
> > device FD is bound, it remains bound until the FD is closed.
> >
> > Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> > Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> > Reviewed-by: Eric Auger <eric.auger@redhat.com>
> > Tested-by: Terrence Xu <terrence.xu@intel.com>
> > Tested-by: Nicolin Chen <nicolinc@nvidia.com>
> > Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
> > Tested-by: Yanting Jiang <yanting.jiang@intel.com>
> > Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
> > Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> > ---
> > drivers/vfio/group.c | 11 ++++++++++-
> > drivers/vfio/vfio.h | 1 +
> > drivers/vfio/vfio_main.c | 16 ++++++++++++++++
> > 3 files changed, 27 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> > index caf53716ddb2..088dd34c8931 100644
> > --- a/drivers/vfio/group.c
> > +++ b/drivers/vfio/group.c
> > @@ -194,9 +194,18 @@ static int vfio_df_group_open(struct vfio_device_file *df)
> > df->iommufd = device->group->iommufd;
> >
> > ret = vfio_df_open(df);
> > - if (ret)
> > + if (ret) {
> > df->iommufd = NULL;
> > + goto out_put_kvm;
> > + }
> > +
> > + /*
> > + * Paired with smp_load_acquire() in vfio_device_fops::ioctl/
> > + * read/write/mmap and vfio_file_has_device_access()
> > + */
> > + smp_store_release(&df->access_granted, true);
> >
> > +out_put_kvm:
> > if (device->open_count == 0)
> > vfio_device_put_kvm(device);
> >
> > diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> > index f9eb52eb9ed7..fdf2fc73f880 100644
> > --- a/drivers/vfio/vfio.h
> > +++ b/drivers/vfio/vfio.h
> > @@ -18,6 +18,7 @@ struct vfio_container;
> >
> > struct vfio_device_file {
> > struct vfio_device *device;
> > + bool access_granted;
>
> Should we make this a more strongly defined data type and later move
> devid (u32) here to partially fill the hole created?

Before your question, let me describe how I place the fields
of this structure to see if it is common practice. The first two
fields are static, so they are in the beginning. The access_granted
is lockless and other fields are protected by locks. So I tried to
put the lock and the fields it protects closely. So this is why I put
devid behind iommufd as both are protected by the same lock.

struct vfio_device_file {
struct vfio_device *device;
struct vfio_group *group;

bool access_granted;
spinlock_t kvm_ref_lock; /* protect kvm field */
struct kvm *kvm;
struct iommufd_ctx *iommufd; /* protected by struct vfio_device_set::lock */
u32 devid; /* only valid when iommufd is valid */
};

>
> I think this is being placed towards the front of the data structure
> for cache line locality given this is a hot path for file operations.
> But bool types have an implementation dependent size, making them
> difficult to pack. Also there will be a tendency to want to make this
> a bit field, which is probably not compatible with the smp lockless
> operations being used here. We might get in front of these issues if
> we just define it as a u8 now. Thanks,

Not quite get why bit field is going to be incompatible with smp
lockless operations. Could you elaborate a bit? And should I define
the access_granted as u8 or "u8:1"?

Regards,
Yi Liu

>
> > spinlock_t kvm_ref_lock; /* protect kvm field */
> > struct kvm *kvm;
> > struct iommufd_ctx *iommufd; /* protected by struct vfio_device_set::lock */
> > diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> > index a3c5817fc545..4c8b7713dc3d 100644
> > --- a/drivers/vfio/vfio_main.c
> > +++ b/drivers/vfio/vfio_main.c
> > @@ -1129,6 +1129,10 @@ static long vfio_device_fops_unl_ioctl(struct file *filep,
> > struct vfio_device *device = df->device;
> > int ret;
> >
> > + /* Paired with smp_store_release() following vfio_df_open() */
> > + if (!smp_load_acquire(&df->access_granted))
> > + return -EINVAL;
> > +
> > ret = vfio_device_pm_runtime_get(device);
> > if (ret)
> > return ret;
> > @@ -1156,6 +1160,10 @@ static ssize_t vfio_device_fops_read(struct file *filep, char
> __user *buf,
> > struct vfio_device_file *df = filep->private_data;
> > struct vfio_device *device = df->device;
> >
> > + /* Paired with smp_store_release() following vfio_df_open() */
> > + if (!smp_load_acquire(&df->access_granted))
> > + return -EINVAL;
> > +
> > if (unlikely(!device->ops->read))
> > return -EINVAL;
> >
> > @@ -1169,6 +1177,10 @@ static ssize_t vfio_device_fops_write(struct file *filep,
> > struct vfio_device_file *df = filep->private_data;
> > struct vfio_device *device = df->device;
> >
> > + /* Paired with smp_store_release() following vfio_df_open() */
> > + if (!smp_load_acquire(&df->access_granted))
> > + return -EINVAL;
> > +
> > if (unlikely(!device->ops->write))
> > return -EINVAL;
> >
> > @@ -1180,6 +1192,10 @@ static int vfio_device_fops_mmap(struct file *filep, struct
> vm_area_struct *vma)
> > struct vfio_device_file *df = filep->private_data;
> > struct vfio_device *device = df->device;
> >
> > + /* Paired with smp_store_release() following vfio_df_open() */
> > + if (!smp_load_acquire(&df->access_granted))
> > + return -EINVAL;
> > +
> > if (unlikely(!device->ops->mmap))
> > return -EINVAL;
> >