Re: [Qemu-devel] [PATCH v6 00/16] KVM platform device passthrough

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2014-09-11 at 15:23 -0700, Christoffer Dall wrote:
> On Thu, Sep 11, 2014 at 04:14:09PM -0600, Alex Williamson wrote:
> > On Tue, 2014-09-09 at 08:31 +0100, Eric Auger wrote:
> > > This RFC series aims at enabling KVM platform device passthrough.
> > > It implements a VFIO platform device, derived from VFIO PCI device.
> > > 
> > > The VFIO platform device uses the host VFIO platform driver which must
> > > be bound to the assigned device prior to the QEMU system start.
> > > 
> > > - the guest can directly access the device register space
> > > - assigned device IRQs are transparently routed to the guest by
> > >   QEMU/KVM (3 methods currently are supported: user-level eventfd
> > >   handling, irqfd, forwarded IRQs)
> > > - iommu is transparently programmed to prevent the device from
> > >   accessing physical pages outside of the guest address space
> > > 
> > > This patch series is made of the following patch files:
> > > 
> > > 1-7) Modifications to PCI code to prepare for VFIO platform device
> > > 8) split of PCI specific code and generic code (move)
> > > 9-11) creation of the VFIO calxeda xgmac platform device, without irqfd
> > >       support (MMIO direct access and IRQ assignment).
> > > 12) fake injection test modality (to test multiple IRQ)
> > > 13) addition of irqfd/virqfd support
> > > 14-16) forwarded IRQ
> > > 
> > > Dependency List:
> > > 
> > > QEMU dependencies:
> > > [1] [PATCH v2 0/9] Dynamic sysbus device allocation support, Alex Graf
> > >     http://lists.gnu.org/archive/html/qemu-ppc/2014-07/msg00047.html
> > > [2] [RFC v3] machvirt dynamic sysbus device instantiation, Eric Auger
> > > [3] [PATCH v2 0/2] actual checks of KVM_CAP_IRQFD and KVM_CAP_IRQFD_RESAMPLE,
> > >     Eric Auger
> > >     http://lists.nongnu.org/archive/html/qemu-devel/2014-09/msg00589.html
> > > [4] [RFC] vfio: migration to trace points, Eric Auger
> > >     http://lists.nongnu.org/archive/html/qemu-devel/2014-09/msg00569.html
> > > 
> > > Kernel Dependencies:
> > > [5] [RFC Patch v6 0/20] VFIO support for platform devices, Antonios Motakis
> > >     https://www.mail-archive.com/kvm@vger.kernel.org/msg103247.html
> > > [6] [PATCH v3] ARM: KVM: add irqfd support, Eric Auger
> > >     https://lkml.org/lkml/2014/9/1/141
> > > [7] arm/arm64: KVM: Various VGIC cleanups and improvements, Christoffer Dall
> > >     http://comments.gmane.org/gmane.linux.ports.arm.kernel/340430
> > > [8] [RFC v2 0/9] KVM-VFIO IRQ forward control, Eric Auger
> > >     https://lkml.org/lkml/2014/9/1/344
> > > [9] [RFC PATCH 0/9] ARM: Forwarding physical interrupts to a guest VM,
> > >     Marc Zyngier
> > >     http://lwn.net/Articles/603514/
> > > 
> > > kernel pieces can be found at:
> > > http://git.linaro.org/people/eric.auger/linux.git
> > > (branch 3.17rc3_irqfd_forward_integ_v2)
> > > QEMU pieces can be found at:
> > > http://git.linaro.org/people/eric.auger/qemu.git (branch vfio_integ_v6)
> > > 
> > > The patch series was tested on Calxeda Midway (ARMv7) where one xgmac
> > > is assigned to KVM host while the second one is assigned to the guest.
> > > Reworked PCI device is not tested.
> > > 
> > > Wiki for Calxeda Midway setup:
> > > https://wiki.linaro.org/LEG/Engineering/Virtualization/Platform_Device_Passthrough_on_Midway
> > > 
> > > History:
> > > 
> > > v5->v6:
> > > - rebase on 2.1rc5 PCI code
> > > - forwarded IRQ first integraton
> > 
> > Why?  Are there acceleration paths that you're concerned cannot be
> > implemented or we do not already have a proof of concept for?  The base
> > kernel patch series you depend on is 3 months old yet this series
> > continues to grow and add new dependencies.  Please let's prioritize
> > getting something upstream instead of adding more blockers to prevent
> > that.  Thanks,
> > 
> I'm not exactly sure what this changelog line was referring to
> (depending on Marc's forwarding IRQ patches?), but just want to add that
> there are a number of dependencies for the GIC that need to go in as
> well (should happen within a few weeks), but I think it's unlikely that
> the IRQ forwarding stuff goes in for v3.18 at this point.
> 
> It may make sense as you suggest to keep that part out of this patch set
> and something merged sooner as opposed to later, but I'm too jet-lagged
> to completely understand if that's going to be a horrible mess.

The point is that we're on v6 of a patch series and its first non-RFC
posting and we're rolling in a first pass at a QEMU implementation that
depends on a contested kernel RFC, which depends on another stagnant
kernel RFC.  I'm fine with working on it in parallel, but give me some
light at the end of the tunnel as a reviewer and maintainer that this
code isn't going to live indefinitely on the mailing list.  Do we really
need those GIC patches do be able to have non-KVM accelerated VFIO
platform device assignment?  We certainly don't need IRQ forwarding.
Thanks,

Alex