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

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 13 Jun 2023 14:36:14 +0000
"Liu, Yi L" <yi.l.liu@intel.com> wrote:

> > From: Alex Williamson <alex.williamson@redhat.com>
> > Sent: Tuesday, June 13, 2023 10:17 PM
> > 
> > On Tue, 13 Jun 2023 05:46:32 +0000
> > "Liu, Yi L" <yi.l.liu@intel.com> wrote:
> >   
> > > > 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.  
> > 
> > I think the primary considerations are locality and compactness.  Hot
> > paths data should be within the first cache line of the structure,
> > related data should share a cache line, and we should use the space
> > efficiently.  What you describe seems largely an aesthetic concern,
> > which was not evident to me by the segmentation alone.  
> 
> Sure.
> 
> >   
> > > 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"?  
> > 
> > Perhaps FUD on my part, but load-acquire type operations have specific
> > semantics and it's not clear to me that they interest with compiler
> > generated bit operations.  Thanks,  
> 
> I see. How about below? 
> 
> struct vfio_device_file {
>         struct vfio_device *device;
>         struct vfio_group *group;
>         u8 access_granted;
>         u32 devid; /* only valid when iommufd is valid */
>         spinlock_t kvm_ref_lock; /* protect kvm field */
>         struct kvm *kvm;
>         struct iommufd_ctx *iommufd; /* protected by struct vfio_device_set::lock */
> };

Yep, that's essentially what I was suggesting.  Thanks,

Alex