[PATCH] kvmtool: don't use PCI config space IRQ line field

[Andre Przywara <andre.przywara@arm.com>]

In PCI config space there is an interrupt line field (offset 0x3f),
which is used to initially communicate the IRQ line number from
firmware to the OS. _Hardware_ should never use this information,
as the OS is free to write any information in there.
But kvmtool uses this number when it triggers IRQs in the guest,
which fails starting with Linux 3.19-rc1, where the PCI layer starts
writing the virtual IRQ number in there.

Fix that by storing the IRQ number in a separate field in
struct virtio_pci, which is independent from the PCI config space
and cannot be influenced by the guest.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
---
Hi,

this fixes the hangs we have seen with kvmtool and PCI in 3.19-rc1+.
I scanned kvmtool's code for further usage of that config space
field, but couldn't find anything relevant except pci-shmem.c, which
is completely broken atm, so I didn't bother to fix this.

Cheers,
Andre.

 tools/kvm/include/kvm/virtio-pci.h |    8 ++++++++
 tools/kvm/virtio/pci.c             |    9 ++++++---
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/tools/kvm/include/kvm/virtio-pci.h b/tools/kvm/include/kvm/virtio-pci.h
index c795ce7..b70cadd 100644
--- a/tools/kvm/include/kvm/virtio-pci.h
+++ b/tools/kvm/include/kvm/virtio-pci.h
@@ -30,6 +30,14 @@ struct virtio_pci {
 	u8			isr;
 	u32			features;
 
+	/*
+	 * We cannot rely on the INTERRUPT_LINE byte in the config space once
+	 * we have run guest code, as the OS is allowed to use that field
+	 * as a scratch pad to communicate between driver and PCI layer.
+	 * So store our legacy interrupt line number in here for internal use.
+	 */
+	u8			legacy_irq_line;
+
 	/* MSI-X */
 	u16			config_vector;
 	u32			config_gsi;
diff --git a/tools/kvm/virtio/pci.c b/tools/kvm/virtio/pci.c
index 7556239..e17e5a9 100644
--- a/tools/kvm/virtio/pci.c
+++ b/tools/kvm/virtio/pci.c
@@ -141,7 +141,7 @@ static bool virtio_pci__io_in(struct ioport *ioport, struct kvm_cpu *vcpu, u16 p
 		break;
 	case VIRTIO_PCI_ISR:
 		ioport__write8(data, vpci->isr);
-		kvm__irq_line(kvm, vpci->pci_hdr.irq_line, VIRTIO_IRQ_LOW);
+		kvm__irq_line(kvm, vpci->legacy_irq_line, VIRTIO_IRQ_LOW);
 		vpci->isr = VIRTIO_IRQ_LOW;
 		break;
 	default:
@@ -299,7 +299,7 @@ int virtio_pci__signal_vq(struct kvm *kvm, struct virtio_device *vdev, u32 vq)
 			kvm__irq_trigger(kvm, vpci->gsis[vq]);
 	} else {
 		vpci->isr = VIRTIO_IRQ_HIGH;
-		kvm__irq_trigger(kvm, vpci->pci_hdr.irq_line);
+		kvm__irq_trigger(kvm, vpci->legacy_irq_line);
 	}
 	return 0;
 }
@@ -323,7 +323,7 @@ int virtio_pci__signal_config(struct kvm *kvm, struct virtio_device *vdev)
 			kvm__irq_trigger(kvm, vpci->config_gsi);
 	} else {
 		vpci->isr = VIRTIO_PCI_ISR_CONFIG;
-		kvm__irq_trigger(kvm, vpci->pci_hdr.irq_line);
+		kvm__irq_trigger(kvm, vpci->legacy_irq_line);
 	}
 
 	return 0;
@@ -422,6 +422,9 @@ int virtio_pci__init(struct kvm *kvm, void *dev, struct virtio_device *vdev,
 	if (r < 0)
 		goto free_msix_mmio;
 
+	/* save the IRQ that device__register() has allocated */
+	vpci->legacy_irq_line = vpci->pci_hdr.irq_line;
+
 	return 0;
 
 free_msix_mmio:
-- 
1.7.9.5