On Wed, 22 Sep 2021 22:34:42 +0000 "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote: > > From: Alex Williamson <alex.williamson@xxxxxxxxxx> > > Sent: Thursday, September 23, 2021 4:11 AM > > > > On Wed, 22 Sep 2021 09:22:52 -0300 > > Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: > > > > > On Wed, Sep 22, 2021 at 09:23:34AM +0000, Tian, Kevin wrote: > > > > > > > > Providing an ioctl to bind to a normal VFIO container or group might > > > > > allow a reasonable fallback in userspace.. > > > > > > > > I didn't get this point though. An error in binding already allows the > > > > user to fall back to the group path. Why do we need introduce another > > > > ioctl to explicitly bind to container via the nongroup interface? > > > > > > New userspace still needs a fallback path if it hits the 'try and > > > fail'. Keeping the device FD open and just using a different ioctl to > > > bind to a container/group FD, which new userspace can then obtain as a > > > fallback, might be OK. > > > > > > Hard to see without going through the qemu parts, so maybe just keep > > > it in mind > > > > If we assume that the container/group/device interface is essentially > > deprecated once we have iommufd, it doesn't make a lot of sense to me > > to tack on a container/device interface just so userspace can avoid > > reverting to the fully legacy interface. > > > > But why would we create vfio device interface files at all if they > > can't work? I'm not really on board with creating a try-and-fail > > interface for a mechanism that cannot work for a given device. The > > existence of the device interface should indicate that it's supported. > > Thanks, > > > > Now it's a try-and-fail model even for devices which support iommufd. > Per Jason's suggestion, a device is always opened with a parked fops > which supports only bind. Binding serves as the contract for handling > exclusive ownership on a device and switching to normal fops if > succeed. So the user has to try-and-fail in case multiple threads attempt > to open a same device. Device which doesn't support iommufd is not > different, except binding request 100% fails (due to missing .bind_iommufd > in kernel driver). That's a rather important difference. I don't really see how that's comparable to the mutually exclusive nature of the legacy vs device interface. We're not going to present a vfio device interface for SW mdevs that can't participate in iommufd, right? Thanks, Alex
