Re: [PATCH v3 2/7] vfio: introduce vfio_dma_rw to read/write a range of IOVAs

[Alex Williamson]

On Thu, 5 Mar 2020 20:21:48 -0500
Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:

> On Mon, Feb 24, 2020 at 04:47:15PM +0800, Zhao, Yan Y wrote:
> > vfio_dma_rw will read/write a range of user space memory pointed to by
> > IOVA into/from a kernel buffer without enforcing pinning the user space
> > memory.
> > 
> > TODO: mark the IOVAs to user space memory dirty if they are written in
> > vfio_dma_rw().
> > 
> > Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> > Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> > ---
> >  drivers/vfio/vfio.c             | 49 +++++++++++++++++++++
> >  drivers/vfio/vfio_iommu_type1.c | 77 +++++++++++++++++++++++++++++++++
> >  include/linux/vfio.h            |  5 +++
> >  3 files changed, 131 insertions(+)
> > 
> > diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> > index 914bdf4b9d73..902867627cbf 100644
> > --- a/drivers/vfio/vfio.c
> > +++ b/drivers/vfio/vfio.c
> > @@ -1998,6 +1998,55 @@ int vfio_unpin_pages(struct device *dev, unsigned long *user_pfn, int npage)
> >  }
> >  EXPORT_SYMBOL(vfio_unpin_pages);
> >  
> > +
> > +/*
> > + * This interface allows the CPUs to perform some sort of virtual DMA on
> > + * behalf of the device.
> > + *
> > + * CPUs read/write a range of IOVAs pointing to user space memory into/from
> > + * a kernel buffer.
> > + *
> > + * As the read/write of user space memory is conducted via the CPUs and is
> > + * not a real device DMA, it is not necessary to pin the user space memory.
> > + *
> > + * The caller needs to call vfio_group_get_external_user() or
> > + * vfio_group_get_external_user_from_dev() prior to calling this interface,
> > + * so as to prevent the VFIO group from disposal in the middle of the call.
> > + * But it can keep the reference to the VFIO group for several calls into
> > + * this interface.
> > + * After finishing using of the VFIO group, the caller needs to release the
> > + * VFIO group by calling vfio_group_put_external_user().
> > + *
> > + * @group [in]: vfio group of a device
> > + * @iova [in] : base IOVA of a user space buffer
> > + * @data [in] : pointer to kernel buffer
> > + * @len [in]  : kernel buffer length
> > + * @write     : indicate read or write
> > + * Return error code on failure or 0 on success.
> > + */
> > +int vfio_dma_rw(struct vfio_group *group, dma_addr_t iova,
> > +		void *data, size_t len, bool write)  
> hi Alex
> May I rename this interface to vfio_dma_rw_from_group() that takes
> VFIO group as arg and add another interface vfio_dma_rw(struct device *dev...) ?
> That might be easier for a driver to use the second one if it does not care about
> performance much.

Perhaps vfio_group_dma_rw() and vfio_dev_dma_rw()?  I'd be reluctant to
add the latter, if a caller doesn't care about performance then they
won't mind making a couple calls to get and release the group reference.
Thanks,

Alex