Re: [RFC v2 0/9] KVM-VFIO IRQ forward control

[Christoffer Dall]

On Tue, Sep 02, 2014 at 03:05:41PM -0600, Alex Williamson wrote:
> On Mon, 2014-09-01 at 14:52 +0200, Eric Auger wrote:
> > This RFC proposes an integration of "ARM: Forwarding physical
> > interrupts to a guest VM" (http://lwn.net/Articles/603514/) in
> > KVM.
> > 
> > It enables to transform a VFIO platform driver IRQ into a forwarded
> > IRQ. The direct benefit is that, for a level sensitive IRQ, a VM
> > switch can be avoided on guest virtual IRQ completion. Before this
> > patch, a maintenance IRQ was triggered on the virtual IRQ completion.
> > 
> > When the IRQ is forwarded, the VFIO platform driver does not need to
> > disable the IRQ anymore. Indeed when returning from the IRQ handler
> > the IRQ is not deactivated. Only its priority is lowered. This means
> > the same IRQ cannot hit before the guest completes the virtual IRQ
> > and the GIC automatically deactivates the corresponding physical IRQ.
> > 
> > Besides, the injection still is based on irqfd triggering. The only
> > impact on irqfd process is resamplefd is not called anymore on
> > virtual IRQ completion since this latter becomes "transparent".
> > 
> > The current integration is based on an extension of the KVM-VFIO
> > device, previously used by KVM to interact with VFIO groups. The
> > patch serie now enables KVM to directly interact with a VFIO
> > platform device. The VFIO external API was extended for that purpose.
> > 
> > Th KVM-VFIO device can get/put the vfio platform device, check its
> > integrity and type, get the IRQ number associated to an IRQ index.
> > 
> > The IRQ forward programming is architecture specific (virtual interrupt
> > controller programming basically). However the whole infrastructure is
> > kept generic.
> > 
> > from a user point of view, the functionality is provided through new
> > KVM-VFIO device commands, KVM_DEV_VFIO_DEVICE_(UN)FORWARD_IRQ
> > and the capability can be checked with KVM_HAS_DEVICE_ATTR.
> > Assignment can only be changed when the physical IRQ is not active.
> > It is the responsability of the user to do this check.
> > 
> > This patch serie has the following dependencies:
> > - "ARM: Forwarding physical interrupts to a guest VM"
> >   (http://lwn.net/Articles/603514/) in
> > - [PATCH v3] irqfd for ARM
> > - and obviously the VFIO platform driver serie:
> >   [RFC PATCH v6 00/20] VFIO support for platform devices on ARM
> >   https://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/msg103247.html
> > 
> > Integrated pieces can be found at
> > ssh://git.linaro.org/people/eric.auger/linux.git
> > on branch 3.17rc3_irqfd_forward_integ_v2
> > 
> > This was was tested on Calxeda Midway, assigning the xgmac main IRQ.
> > 
> > v1 -> v2:
> > - forward control is moved from architecture specific file into generic
> >   vfio.c module.
> >   only kvm_arch_set_fwd_state remains architecture specific
> > - integrate Kim's patch which enables KVM-VFIO for ARM
> > - fix vgic state bypass in vgic_queue_hwirq
> > - struct kvm_arch_forwarded_irq moved from arch/arm/include/uapi/asm/kvm.h
> >   to include/uapi/linux/kvm.h
> >   also irq_index renamed into index and guest_irq renamed into gsi
> > - ASSIGN/DEASSIGN renamed into FORWARD/UNFORWARD
> > - vfio_external_get_base_device renamed into vfio_external_base_device
> > - vfio_external_get_type removed
> > - kvm_vfio_external_get_base_device renamed into kvm_vfio_external_base_device
> > - __KVM_HAVE_ARCH_KVM_VFIO renamed into __KVM_HAVE_ARCH_KVM_VFIO_FORWARD
> > 
> > Eric Auger (8):
> >   KVM: ARM: VGIC: fix multiple injection of level sensitive forwarded
> >     IRQ
> >   KVM: ARM: VGIC: add forwarded irq rbtree lock
> >   VFIO: platform: handler tests whether the IRQ is forwarded
> >   KVM: KVM-VFIO: update user API to program forwarded IRQ
> >   VFIO: Extend external user API
> >   KVM: KVM-VFIO: add new VFIO external API hooks
> >   KVM: KVM-VFIO: generic KVM_DEV_VFIO_DEVICE command and IRQ forwarding
> >     control
> >   KVM: KVM-VFIO: ARM forwarding control
> > 
> > Kim Phillips (1):
> >   ARM: KVM: Enable the KVM-VFIO device
> > 
> >  Documentation/virtual/kvm/devices/vfio.txt |  26 ++
> >  arch/arm/include/asm/kvm_host.h            |   7 +
> >  arch/arm/kvm/Kconfig                       |   1 +
> >  arch/arm/kvm/Makefile                      |   4 +-
> >  arch/arm/kvm/kvm_vfio_arm.c                |  85 +++++
> >  drivers/vfio/platform/vfio_platform_irq.c  |   7 +-
> >  drivers/vfio/vfio.c                        |  24 ++
> >  include/kvm/arm_vgic.h                     |   1 +
> >  include/linux/kvm_host.h                   |  27 ++
> >  include/linux/vfio.h                       |   3 +
> >  include/uapi/linux/kvm.h                   |   9 +
> >  virt/kvm/arm/vgic.c                        |  59 +++-
> >  virt/kvm/vfio.c                            | 497 ++++++++++++++++++++++++++++-
> >  13 files changed, 733 insertions(+), 17 deletions(-)
> >  create mode 100644 arch/arm/kvm/kvm_vfio_arm.c
> > 
> 
> Have we ventured too far in the other direction?  I suppose what I was
> hoping to see was something more like:
> 
> 	case KVM_DEV_VFIO_DEVICE_FORWARD_IRQ:{
> 
> 		/* get vfio_device */
> 
> 		/* get mutex */
> 
> 		/* verify device+irq isn't already forwarded */
> 
> 		/* allocate device/forwarded irq */
> 
> 		/* get struct device */
> 
> 		/* callout to arch code passing struct device, gsi, ... */
> 
> 		/* if success, add to kv, else free and error */
> 
> 		/* mutex unlock */
> 	}

I think that's essentially what this patch set is trying to do, but
there are just too many complicated intertwining cases right now that
makes the code hard to read.

> 
> Exposing the internal mutex out to arch code, as in v1, was an
> indication that we were pushing too much out to arch code, but including
> platform_device.h into virt/kvm/vfio.c tells me we're still not
> abstracting at the right point.  Thanks,
> 
I raised my eyebrows over the platform device bus thingy here as well,
but on the other hand, there's nothing ARM-specific about referring to
the platform device bus.

I think perhaps it just has to be made more clear that the generic code
deals with translating the device resources in the necessary way, and
currently it only supports vfio-platform devices?

-Christoffer
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