RE: [PATCH RFC v2 00/18] Add VFIO mediated device support and DEV-MSI support for the idxd driver
From: Tian, Kevin
Date: Tue Aug 11 2020 - 21:58:55 EST
> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Wednesday, August 12, 2020 1:01 AM
> On Mon, 10 Aug 2020 07:32:24 +0000
> "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > Sent: Friday, August 7, 2020 8:20 PM
> > >
> > > On Wed, Aug 05, 2020 at 07:22:58PM -0600, Alex Williamson wrote:
> > >
> > > > If you see this as an abuse of the framework, then let's identify those
> > > > specific issues and come up with a better approach. As we've discussed
> > > > before, things like basic PCI config space emulation are acceptable
> > > > overhead and low risk (imo) and some degree of register emulation is
> > > > well within the territory of an mdev driver.
> > >
> > > What troubles me is that idxd already has a direct userspace interface
> > > to its HW, and does userspace DMA. The purpose of this mdev is to
> > > provide a second direct userspace interface that is a little different
> > > and trivially plugs into the virtualization stack.
> > No. Userspace DMA and subdevice passthrough (what mdev provides)
> > are two distinct usages IMO (at least in idxd context). and this might
> > be the main divergence between us, thus let me put more words here.
> > If we could reach consensus in this matter, which direction to go
> > would be clearer.
> > First, a passthrough interface requires some unique requirements
> > which are not commonly observed in an userspace DMA interface, e.g.:
> > - Tracking DMA dirty pages for live migration;
> > - A set of interfaces for using SVA inside guest;
> > * PASID allocation/free (on some platforms);
> > * bind/unbind guest mm/page table (nested translation);
> > * invalidate IOMMU cache/iotlb for guest page table changes;
> > * report page request from device to guest;
> > * forward page response from guest to device;
> > - Configuring irqbypass for posted interrupt;
> > - ...
> > Second, a passthrough interface requires delegating raw controllability
> > of subdevice to guest driver, while the same delegation might not be
> > required for implementing an userspace DMA interface (especially for
> > modern devices which support SVA). For example, idxd allows following
> > setting per wq (guest driver may configure them in any combination):
> > - put in dedicated or shared mode;
> > - enable/disable SVA;
> > - Associate guest-provided PASID to MSI/IMS entry;
> > - set threshold;
> > - allow/deny privileged access;
> > - allocate/free interrupt handle (enlightened for guest);
> > - collect error status;
> > - ...
> > We plan to support idxd userspace DMA with SVA. The driver just needs
> > to prepare a wq with a predefined configuration (e.g. shared, SVA,
> > etc.), bind the process mm to IOMMU (non-nested) and then map
> > the portal to userspace. The goal that userspace can do DMA to
> > associated wq doesn't change the fact that the wq is still *owned*
> > and *controlled* by kernel driver. However as far as passthrough
> > is concerned, the wq is considered 'owned' by the guest driver thus
> > we need an interface which can support low-level *controllability*
> > from guest driver. It is sort of a mess in uAPI when mixing the
> > two together.
> > Based on above two reasons, we see distinct requirements between
> > userspace DMA and passthrough interfaces, at least in idxd context
> > (though other devices may have less distinction in-between). Therefore,
> > we didn't see the value/necessity of reinventing the wheel that mdev
> > already handles well to evolve an simple application-oriented usespace
> > DMA interface to a complex guest-driver-oriented passthrough interface.
> > The complexity of doing so would incur far more kernel-side changes
> > than the portion of emulation code that you've been concerned about...
> > >
> > > I don't think VFIO should be the only entry point to
> > > virtualization. If we say the universe of devices doing user space DMA
> > > must also implement a VFIO mdev to plug into virtualization then it
> > > will be alot of mdevs.
> > Certainly VFIO will not be the only entry point. and This has to be a
> > case-by-case decision. If an userspace DMA interface can be easily
> > adapted to be a passthrough one, it might be the choice. But for idxd,
> > we see mdev a much better fit here, given the big difference between
> > what userspace DMA requires and what guest driver requires in this hw.
> > >
> > > I would prefer to see that the existing userspace interface have the
> > > extra needed bits for virtualization (eg by having appropriate
> > > internal kernel APIs to make this easy) and all the emulation to build
> > > the synthetic PCI device be done in userspace.
> > In the end what decides the direction is the amount of changes that
> > we have to put in kernel, not whether we call it 'emulation'. For idxd,
> > adding special passthrough requirements (guest SVA, dirty tracking,
> > etc.) and raw controllability to the simple userspace DMA interface
> > is for sure making kernel more complex than reusing the mdev
> > framework (plus some degree of emulation mockup behind). Not to
> > mention the merit of uAPI compatibility with mdev...
> I agree with a lot of this argument, exposing a device through a
> userspace interface versus allowing user access to a device through a
> userspace interface are different levels of abstraction and control.
> In an ideal world, perhaps we could compose one from the other, but I
> don't think the existence of one is proof that the other is redundant.
> That's not to say that mdev/vfio isn't ripe for abuse in this space,
> but I'm afraid the test for that abuse is probably much more subtle.
> I'll also remind folks that LPC is coming up in just a couple short
> weeks and this might be something we should discuss (virtually)
> in-person. uconf CfPs are currently open. </plug> Thanks,
Yes, LPC is a good place to reach consensus. btw I saw there is
already one VFIO topic called "device assignment/sub-assignment".
Do you think whether this can be covered under that topic, or
makes more sense to be a new one?