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

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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

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