linux-kernel - [PATCH] virtio_pci: read interrupt pin directly

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Date:   Wed, 12 Oct 2022 07:07:40 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Angus Chen <angus.chen@...uarmicro.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org
Subject: [PATCH] virtio_pci: read interrupt pin directly

commit 71491c54eafa ("virtio_pci: don't try to use intxif pin is zero")
breaks virtio_pci on powerpc, when running as a qemu guest.

vp_find_vqs() bails out because pci_dev->pin == 0.

But pci_dev->irq is populated correctly, so vp_find_vqs_intx() would
succeed if we called it - which is what the code used to do.

This seems to happen because pci_dev->pin is not populated in
pci_assign_irq().

Which is absolutely a bug in the relevant PCI code, but it
may also affect other platforms that use of_irq_parse_and_map_pci().

Work around the issue in virtio for now, and let's try to fix
all affected pci systems and then we can revert this.

Reported-by: Michael Ellerman <mpe@...erman.id.au>
Fixes: 71491c54eafa ("virtio_pci: don't try to use intxif pin is zero")
Cc: "Angus Chen" <angus.chen@...uarmicro.com>
Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 drivers/virtio/virtio_pci_common.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
index 4df77eeb4d16..6155ea4e7e4b 100644
--- a/drivers/virtio/virtio_pci_common.c
+++ b/drivers/virtio/virtio_pci_common.c
@@ -400,6 +400,7 @@ int vp_find_vqs(struct virtio_device *vdev, unsigned int nvqs,
 		struct irq_affinity *desc)
 {
 	int err;
+	u8 pin = 0;
 
 	/* Try MSI-X with one vector per queue. */
 	err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names, true, ctx, desc);
@@ -409,8 +410,13 @@ int vp_find_vqs(struct virtio_device *vdev, unsigned int nvqs,
 	err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names, false, ctx, desc);
 	if (!err)
 		return 0;
-	/* Is there an interrupt pin? If not give up. */
-	if (!(to_vp_device(vdev)->pci_dev->pin))
+	/*
+	 * Is there an interrupt pin? If not give up.
+	 * NB: It would seem to be better to use pci_dev->pin - unfortunately
+	 * not all platforms populate it.
+	 */
+	pci_read_config_byte(dev, PCI_INTERRUPT_PIN, &pin);
+	if (!pin)
 		return err;
 	/* Finally fall back to regular interrupts. */
 	return vp_find_vqs_intx(vdev, nvqs, vqs, callbacks, names, ctx);
-- 
MST