Re: [PATCH v4 9/9] KVM: Allow host IRQ sharing for passed-through PCI 2.3 devices

Avi Kivity
2010-11-09T13:28:00Z

On 11/08/2010 01:21 PM, Jan Kiszka wrote:
> PCI 2.3 allows to generically disable IRQ sources at device level. This
> enables us to share IRQs of such devices between on the host side when
> passing them to a guest. This feature is optional, user space has to
> request it explicitly. Moreover, user space can inform us about its view
> of PCI_COMMAND_INTX_DISABLE so that we can avoid unmasking the interrupt
> and signaling it if the guest masked it via the PCI config space.
>

It's a pity this cannot be done transparently. We could detect multiple
devices sharing the line, but what about PCI_COMMAND_INTX_DISABLE?

Perhaps we can hook the kernel's handler for this bit?

>
> /* 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, thus enable IRQ sharing with other
> +assigned devices or host devices. 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];
> };
>
> +5.54 KVM_ASSIGN_SET_INTX_MASK

4.54?

(54? wow.)

> +
> +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 if userspace lies?

> +
> +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.
> +
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index fe83eb0..7f1627c 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -468,6 +468,7 @@ struct kvm_assigned_dev_kernel {
> unsigned int entries_nr;
> int host_irq;
> bool host_irq_disabled;
> + bool pci_2_3;
> struct msix_entry *host_msix_entries;
> int guest_irq;
> struct msix_entry *guest_msix_entries;
> @@ -477,6 +478,7 @@ struct kvm_assigned_dev_kernel {
> struct pci_dev *dev;
> struct kvm *kvm;
> spinlock_t intx_lock;
> + struct mutex intx_mask_lock;
> char irq_name[32];
> };

I saw no reason this can't be a spinlock, but perhaps I missed
something. This would allow us to avoid srcu, which is slightly more
expensive than rcu. Since pci 2.3 assigned devices are not a major use
case, I'd like not to penalize the mainstream users for this.

This patch undoes some of the niceness of the previous patches, but I
have no alternative to suggest.

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