linux-kernel - Re: [PATCH 5/5] KVM: Allow host IRQ sharing for passed-through PCI 2.3 devices

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Date:	Mon, 06 Dec 2010 18:21:37 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jan Kiszka <jan.kiszka@....de>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel@...r.kernel.org, kvm <kvm@...r.kernel.org>,
	Tom Lyon <pugs@...co.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jan Kiszka <jan.kiszka@...mens.com>
Subject: Re: [PATCH 5/5] KVM: Allow host IRQ sharing for passed-through PCI
 2.3 devices

On 12/04/2010 01:37 AM, Jan Kiszka wrote:
> From: Jan Kiszka<jan.kiszka@...mens.com>
>
> PCI 2.3 allows to generically disable IRQ sources at device level. This
> enables us to share IRQs of such devices on the host side when passing
> them to a guest.
>
> However, IRQ disabling via the PCI config space is more costly than
> masking the line via disable_irq. Therefore we register an IRQ sharing
> notifier and switch between line and device level disabling on demand.
>
> This feature is optional, user space has to request it explicitly as it
> also has to inform us about its view of PCI_COMMAND_INTX_DISABLE. That
> way, we can avoid unmasking the interrupt and signaling it if the guest
> masked it via the PCI config space.
>
>
> diff --git a/Documentation/kvm/api.txt b/Documentation/kvm/api.txt
> index e1a9297..5e164db 100644
> --- a/Documentation/kvm/api.txt
> +++ b/Documentation/kvm/api.txt
> @@ -1112,6 +1112,14 @@ following flags are specified:
>
>   /* Depends on KVM_CAP_IOMMU */
>   #define KVM_DEV_ASSIGN_ENABLE_IOMMU	(1<<  0)
> +/* The following two depend on KVM_CAP_PCI_2_3 */
> +#define KVM_DEV_ASSIGN_PCI_2_3		(1<<  1)
> +#define KVM_DEV_ASSIGN_MASK_INTX	(1<<  2)
> +
> +If KVM_DEV_ASSIGN_PCI_2_3 is set, the kernel will manage legacy INTx interrupts
> +via the PCI-2.3-compliant device-level mask, but only if IRQ sharing with other
> +assigned or host devices requires it. KVM_DEV_ASSIGN_MASK_INTX specifies the
> +guest's view on the INTx mask, see KVM_ASSIGN_SET_INTX_MASK for details.
>
>   4.48 KVM_DEASSIGN_PCI_DEVICE
>
> @@ -1263,6 +1271,23 @@ struct kvm_assigned_msix_entry {
>   	__u16 padding[3];
>   };
>
> +4.54 KVM_ASSIGN_SET_INTX_MASK
> +
> +Capability: KVM_CAP_PCI_2_3
> +Architectures: x86
> +Type: vm ioctl
> +Parameters: struct kvm_assigned_pci_dev (in)
> +Returns: 0 on success, -1 on error
> +
> +Informs the kernel about the guest's view on the INTx mask. As long as the
> +guest masks the legacy INTx, the kernel will refrain from unmasking it at
> +hardware level and will not assert the guest's IRQ line. User space is still
> +responsible for applying this state to the assigned device's real config space.

What's the protocol for doing this?  I suppose userspace has to disable 
interrupts, ioctl(SET_INTX_MASK, masked), ..., ioctl(SET_INTX_MASK, 
unmasked), enable interrupts?

Isn't there a race window between the two operations?

Maybe we should give the kernel full ownership of that bit.

> +
> +See KVM_ASSIGN_DEV_IRQ for the data structure. The target device is specified
> +by assigned_dev_id. In the flags field, only KVM_DEV_ASSIGN_MASK_INTX is
> +evaluated.
> +

-- 
error compiling committee.c: too many arguments to function

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