On Tue, 8 Sep 2020 15:13:32 +0800 Xu Yilun <yilun.xu@xxxxxxxxx> wrote: > This patch adds description for VFIO Mdev support for dfl devices on > dfl bus. > > Signed-off-by: Xu Yilun <yilun.xu@xxxxxxxxx> > Signed-off-by: Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx> > --- > Documentation/fpga/dfl.rst | 20 ++++++++++++++++++++ > 1 file changed, 20 insertions(+) > > diff --git a/Documentation/fpga/dfl.rst b/Documentation/fpga/dfl.rst > index 0404fe6..f077754 100644 > --- a/Documentation/fpga/dfl.rst > +++ b/Documentation/fpga/dfl.rst > @@ -502,6 +502,26 @@ FME Partial Reconfiguration Sub Feature driver (see drivers/fpga/dfl-fme-pr.c) > could be a reference. > > > +VFIO Mdev support for DFL devices > +================================= > +As we introduced a dfl bus for private features, they could be added to dfl bus > +as independent dfl devices. There is a requirement to handle these devices > +either by kernel drivers or by direct access from userspace. Usually we bind > +the kernel drivers to devices which provide board management functions, and > +gives user direct access to devices which cooperate closely with user > +controlled Accelerated Function Unit (AFU). We realize this with a VFIO Mdev > +implementation. When we bind the vfio-mdev-dfl driver to a dfl device, it > +realizes a group of callbacks and registers to the Mdev framework as a > +parent (physical) device. It could then create one (available_instances == 1) > +mdev device. > +Since dfl devices are sub devices of FPGA DFL physical devices (e.g. PCIE > +device), which provide no DMA isolation for each sub device, this may leads to > +DMA isolation problem if a private feature is designed to be capable of DMA. > +The AFU user could potentially access the whole device addressing space and > +impact the private feature. So now the general HW design rule is, no DMA > +capability for private features. It eliminates the DMA isolation problem. What's the advantage of entangling mdev/vfio in this approach versus simply exposing the MMIO region of the device via sysfs (similar to a resource file in pci-sysfs)? This implementation doesn't support interrupts, it doesn't support multiplexing of a device, it doesn't perform any degree of mediation, it seems to simply say "please don't do DMA". I don't think that's acceptable for an mdev driver. If you want to play loose with isolation, do it somewhere else. Thanks, Alex
