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

[Yan Zhao]

On Sat, Mar 07, 2020 at 12:27:46AM +0800, Alex Williamson wrote:
> 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,
>
yes, it makes sense. Then I withdraw this request :)

Thanks
Yan