linux-kernel - Re: [RFC 4/9] KVM: KVM-VFIO: update user API to program forwarded IRQ

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Date:	Tue, 26 Aug 2014 13:01:55 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Eric Auger <eric.auger@...aro.org>
Cc:	eric.auger@...com, christoffer.dall@...aro.org,
	marc.zyngier@....com, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
	joel.schopp@....com, kim.phillips@...escale.com,
	linux-kernel@...r.kernel.org, patches@...aro.org,
	will.deacon@....com, a.motakis@...tualopensystems.com,
	a.rigo@...tualopensystems.com, john.liuli@...wei.com
Subject: Re: [RFC 4/9] KVM: KVM-VFIO: update user API to program forwarded
 IRQ

On Mon, 2014-08-25 at 15:27 +0200, Eric Auger wrote:
> add new device group commands:
> - KVM_DEV_VFIO_DEVICE_ASSIGN_IRQ and
>   KVM_DEV_VFIO_DEVICE_DEASSIGN_IRQ
> 
> which enable to turn forwarded IRQ mode on/off.
> 
> Signed-off-by: Eric Auger <eric.auger@...aro.org>
> ---
>  Documentation/virtual/kvm/devices/vfio.txt | 25 +++++++++++++++++++++++++
>  arch/arm/include/uapi/asm/kvm.h            |  6 ++++++
>  include/uapi/linux/kvm.h                   |  3 +++
>  3 files changed, 34 insertions(+)
> 
> diff --git a/Documentation/virtual/kvm/devices/vfio.txt b/Documentation/virtual/kvm/devices/vfio.txt
> index ef51740..c8b3fa1 100644
> --- a/Documentation/virtual/kvm/devices/vfio.txt
> +++ b/Documentation/virtual/kvm/devices/vfio.txt
> @@ -13,6 +13,7 @@ VFIO-group is held by KVM.
>  
>  Groups:
>    KVM_DEV_VFIO_GROUP
> +  KVM_DEV_VFIO_DEVICE
>  
>  KVM_DEV_VFIO_GROUP attributes:
>    KVM_DEV_VFIO_GROUP_ADD: Add a VFIO group to VFIO-KVM device tracking
> @@ -20,3 +21,27 @@ KVM_DEV_VFIO_GROUP attributes:
>  
>  For each, kvm_device_attr.addr points to an int32_t file descriptor
>  for the VFIO group.
> +
> +KVM_DEV_VFIO_DEVICE attributes:
> +  KVM_DEV_VFIO_DEVICE_ASSIGN_IRQ
> +  KVM_DEV_VFIO_DEVICE_DEASSIGN_IRQ
> +
> +For each, kvm_device_attr.addr points to an kvm_arch_forwarded_irq.
> +This user API makes possible to create a special IRQ handling mode,
> +currently supported only on ARM, where KVM and a VFIO platform driver
> +collaborate to improve IRQ handling performance.
> +fd represents the file descriptor of a valid VFIO device whose physical
> +IRQ, referenced by its irq_index is injected to the VM guest_irq.
> +
> +On ASSIGN_IRQ, KVM-VFIO device programs:
> +- the host, to not complete the physical IRQ itself.
> +- the GIC, to automatically complete the physical IRQ when the guest
> +  completes the virtual IRQ
> +This avoid trapping the end-of-interrupt for level sensitive IRQ.
> +
> +On DEASSIGN_IRQ, one come back to the mode where the host completes the
> +physical IRQ and the guest only completes the virtual IRQ.
> +
> +It is up to the caller of this API to get the assurance the IRQ is not
> +outstanding when the ASSIGN/DEASSIGN is called. This could lead to some
> +inconsistency on who is going to complete the IRQ.

Why not call these FORWARD/UNFORWARD or something since the operation
isn't really doing anything with assignment of the IRQ.  The IRQ is
already "assigned", we're modifying the behavior.

> diff --git a/arch/arm/include/uapi/asm/kvm.h b/arch/arm/include/uapi/asm/kvm.h
> index 3034c66..1920b33 100644
> --- a/arch/arm/include/uapi/asm/kvm.h
> +++ b/arch/arm/include/uapi/asm/kvm.h
> @@ -109,6 +109,12 @@ struct kvm_sync_regs {
>  struct kvm_arch_memory_slot {
>  };
>  
> +struct kvm_arch_forwarded_irq {
> +	__u32 fd; /* file desciptor of the VFIO device */
> +	__u32 irq_index; /* platform device index of the IRQ */

The vfio-platform device IRQ index?  ARM is the only implementation we
have of this, but to keep it generic maybe the comment should read "vfio
device IRQ index".

> +	__u32 guest_irq; /* virtual IRQ number */

This would be a GSI or similar concept if we were on x86.  I'm a little
confused that were using an arch structure here rather than trying to
keep the kvm-vfio device interface neutral.  Maybe it makes sense, I'm
not sure.  Thanks,

Alex

> +};
> +
>  /* If you need to interpret the index values, here is the key: */
>  #define KVM_REG_ARM_COPROC_MASK		0x000000000FFF0000
>  #define KVM_REG_ARM_COPROC_SHIFT	16
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index cf3a2ff..b149ba8 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -954,6 +954,9 @@ struct kvm_device_attr {
>  #define  KVM_DEV_VFIO_GROUP			1
>  #define   KVM_DEV_VFIO_GROUP_ADD			1
>  #define   KVM_DEV_VFIO_GROUP_DEL			2
> +#define  KVM_DEV_VFIO_DEVICE			2
> +#define   KVM_DEV_VFIO_DEVICE_ASSIGN_IRQ			1
> +#define   KVM_DEV_VFIO_DEVICE_DEASSIGN_IRQ			2
>  #define KVM_DEV_TYPE_ARM_VGIC_V2	5
>  #define KVM_DEV_TYPE_FLIC		6
>  



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