[RFC] virtio-iommu version 0.5

Linu Cherian
Tue, 24 Oct 2017 16:58:59 -0000

Hi Jean,
Thanks for your reply.

On Tue Oct 24, 2017 at 09:37:12AM +0100, Jean-Philippe Brucker
wrote:
> Hi Linu,
>
> On 24/10/17 07:27, Linu Cherian wrote:
> > Hi Jean,
> >
> > On Mon Oct 23, 2017 at 10:32:41AM +0100, Jean-Philippe Brucker wrote:
> >> This is version 0.5 of the virtio-iommu specification, the
paravirtualized
> >> IOMMU. This version addresses feedback from v0.4 and adds an event
virtqueue.
> >> Please find the specification, LaTeX sources and pdf, at:
> >> git://linux-arm.org/virtio-iommu.git viommu/v0.5
> >>
http://linux-arm.org/git?p=virtio-iommu.git;a=blob;f=dist/v0.5/virtio-iommu-v0.5.pdf
> >>
> >> A detailed changelog since v0.4 follows. You can find the pdf diff
at:
> >>
http://linux-arm.org/git?p=virtio-iommu.git;a=blob;f=dist/diffs/virtio-iommu-pdf-diff-v0.4-v0.5.pdf
> >>
> >> * Add an event virtqueue for the device to report translation
faults to
> >>   the driver. For the moment only unrecoverable faults are
available but
> >>   future versions will extend it.
> >> * Simplify PROBE request by removing the ack part, and flattening
RESV
> >>   properties.
> >> * Rename "address space" to "domain". The
change might seem futile but
> >>   allows to introduce PASIDs and other features cleanly in the
next
> >>   versions. In the same vein, the few remaining "device"
occurrences were
> >>   replaced by "endpoint", to avoid any confusion with
"the device"
> >>   referring to the virtio device across the document.
> >> * Add implementation notes for RESV_MEM properties.
> >> * Update ACPI table definition.
> >> * Fix typos and clarify a few things.
> >>
> >> I will publish the Linux driver for v0.5 shortly. Then for next
versions
> >> I'll focus on optimizations and adding support for hardware
acceleration.
> >>
> >> Existing implementations are simple and can certainly be
optimized, even
> >> without architectural changes. But the architecture itself can
also be
> >> improved in a number of ways. Currently it is designed to work
well with
> >> VFIO. However, having explicit MAP requests is less efficient*
than page
> >> tables for emulated and PV endpoints, and the current architecture
doesn't
> >> address this. Binding page tables is an obvious way to improve
throughput
> >> in that case, but we can explore cleverer (and possibly simpler)
ways to
> >> do it.
> >>
> >> So first we'll work on getting the base device and driver
merged, then
> >> we'll analyze and compare several ideas for improving
performance.
> >>
> >> Thanks,
> >> Jean
> >>
> >> * I have yet to study this behaviour, and would be interested in
any
> >> prior art on the subject of analyzing devices DMA patterns (virtio
and
> >> others)
> >
> >
> > From the spec,
> > Under future extensions.
> >
> > "Page Table Handover, to allow guests to manage their own page
tables and share them with the MMU"
> >
> > Had few questions on this.
> >
> > 1. Did you mean SVM support for vfio-pci devices attached to guest
processes here.
>
> Yes, using the VFIO BIND and INVALIDATE ioctls that Intel is working on,
> and adding requests in pretty much the same format to virtio-iommu.
>
> > 2. Can you give some hints on how this is going to work , since
virtio-iommu guest kernel
> >    driver need to create stage 1 page table as required by hardware
which is not the case now.
> >    CMIIW.
>
> The virtio-iommu device advertises which PASID/page table format is
> supported by the host (obtained via sysfs and communicated in the PROBE
> request), then the guest binds page tables or PASID tables to a domain and
> populates it. Binding page tables alone is easy because we already have
> the required drivers in the guest (io-pgtable or arch/* for SVM) and code
> in the host to manage PASID tables. But since the PASID table pointer is
> translated by stage-2, it would requires a little more work in the host
> for obtaining GPA buffers from the guest on demand.
  Is this for resolving PCI PRI requests ?.
  IIUC, PCI PRI requests for devices owned by guest need to be resolved
  by guest itself.


 In addition the BIND
> ioctl is different from the one used by VT-d, so this solution didn't
get
> much appreciation.

Could you please share the links on this ?

>
> The alternative is to bind PASID tables.

Sorry, i didnt get the difference here.

It requires to factor the guest
> PASID handling code into a library, which is difficult for SMMU. Luckily
> I'm still working on adding PASID code for SMMUv3, so extracting it out
of
> the driver isn't a big overhead. The good thing about this solution is
> that it reuses any specification work done for VFIO (and vice versa) and
> any host driver change made for vSMMU/VT-d emulations.
>
> Thanks,
> Jean

--
Linu cherian