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

[Alex Williamson]

On Mon, 2014-08-25 at 15:27 +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 KVM-VFIO is extended with an architecture specific implementation.
> IRQ forward control is implemented in the ARM specific part.
> 
> from a user point of view, the functionality is provided through new
> KVM-VFIO device commands, KVM_DEV_VFIO_DEVICE_(DE)ASSIGN_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 v2] irqfd for ARM
>   which itself depends on
>   - arm/arm64: KVM: Various VGIC cleanups and improvements
>     http://lists.infradead.org/pipermail/linux-arm-kernel/2014-June/263685.html
> - 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
> git://git.linaro.org/people/eric.auger/linux.git
> on branch 3.17rc1_forward_integ_v0
> 
> This was was tested on Calxeda Miday, assigning the xgmac main IRQ.

Presumably this optimization should provide lower interrupt exit latency
and lower CPU overhead since we avoid the entire EOI path of the
resampler.  Does it?  It seems like there should be a measurable
improvement with something like netperf TCP_RR with this series.
Thanks,

Alex

> Eric Auger (9):
>   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: allow arch specific implementation
>   KVM: KVM-VFIO: add new VFIO external API hooks
>   KVM: KVM-VFIO: add kvm_vfio_arch_data and accessors
>   KVM: KVM_VFIO: ARM: implement irq forwarding control
> 
>  Documentation/virtual/kvm/devices/vfio.txt |  25 ++
>  arch/arm/include/asm/kvm_host.h            |  16 +
>  arch/arm/include/uapi/asm/kvm.h            |   6 +
>  arch/arm/kvm/Makefile                      |   2 +-
>  arch/arm/kvm/kvm_vfio_arm.c                | 599 +++++++++++++++++++++++++++++
>  drivers/vfio/platform/vfio_platform_irq.c  |   7 +-
>  drivers/vfio/vfio.c                        |  35 ++
>  include/kvm/arm_vgic.h                     |   1 +
>  include/linux/kvm_host.h                   |  30 ++
>  include/linux/vfio.h                       |   4 +
>  include/uapi/linux/kvm.h                   |   3 +
>  virt/kvm/arm/vgic.c                        |  55 ++-
>  virt/kvm/vfio.c                            |  92 +++++
>  13 files changed, 862 insertions(+), 13 deletions(-)
>  create mode 100644 arch/arm/kvm/kvm_vfio_arm.c
> 



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