[PATCH 1/2] virtio_pci_modern_dev: warn once on invalid status

[Abhin Parekadan Jose <abhinjoses@gmail.com>]

vp_modern_get_status() returns the raw device_status byte as read
from the common configuration structure (struct virtio_pci_common_cfg,
mapped via the VIRTIO_PCI_CAP_COMMON_CFG capability). That byte should
only ever contain some combination of the status bits defined by the
virtio spec (bits 0-3, 6-7); bits 4 and 5 are reserved and a
spec-compliant device must never set them. A value with any other bit
set means either the device is violating the spec, or the read never
reached real device state at all -- e.g. because a write of 0x0000 to
the PCI_COMMAND register (config space offset 4) clears the Memory
Space Enable bit, causing the device to stop responding to
memory-mapped register accesses -- effectively simulating an
unresponsive/removed device without a real hot-unplug. In that case
the MMIO read returns the bus's synthesized all-ones response instead
of real device state.

Add VIRTIO_STATUS_ERROR() to the uapi header to recognize such values,
and warn once from vp_modern_get_status() when it sees one, so the
bogus status is visible at its source rather than only showing up as
confusing behavior in callers.

Signed-off-by: Abhin Parekadan Jose <abhinjoses@gmail.com>
---
 drivers/virtio/virtio_pci_modern_dev.c |  8 +++++++-
 include/uapi/linux/virtio_config.h     | 16 ++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_pci_modern_dev.c b/drivers/virtio/virtio_pci_modern_dev.c
index 413a8c353463..60dd8acf1c28 100644
--- a/drivers/virtio/virtio_pci_modern_dev.c
+++ b/drivers/virtio/virtio_pci_modern_dev.c
@@ -480,8 +480,14 @@ EXPORT_SYMBOL_GPL(vp_modern_generation);
 u8 vp_modern_get_status(struct virtio_pci_modern_device *mdev)
 {
 	struct virtio_pci_common_cfg __iomem *cfg = mdev->common;
+	u8 status = vp_ioread8(&cfg->device_status);
 
-	return vp_ioread8(&cfg->device_status);
+	if (VIRTIO_STATUS_ERROR(status)) {
+		WARN_ONCE(1, "virtio: device returned error status: %#x\n",
+			  status);
+	}
+
+	return status;
 }
 EXPORT_SYMBOL_GPL(vp_modern_get_status);
 
diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h
index 2445f365bce7..6f458914c0ba 100644
--- a/include/uapi/linux/virtio_config.h
+++ b/include/uapi/linux/virtio_config.h
@@ -45,6 +45,22 @@
 /* We've given up on this device. */
 #define VIRTIO_CONFIG_S_FAILED		0x80
 
+/*
+ * Check if a status value indicates an error
+ * All device_status bits currently defined by the virtio spec (bits
+ * 0,1,2,3,6,7). Bits 4 and 5 (0x10, 0x20) are reserved/undefined -- a
+ * real device must never set them. A status byte with any bit outside
+ * this mask set cannot be a legitimate value: either the device is
+ * violating the spec, or the read never actually reached it (e.g.
+ * PCI_COMMAND memory decode is disabled and this is a synthesized
+ * all-ones bus response instead of real device state).
+ */
+#define VIRTIO_STATUS_ERROR(val) \
+	(((u8)(val)) & \
+	 ~(VIRTIO_CONFIG_S_ACKNOWLEDGE | VIRTIO_CONFIG_S_DRIVER | \
+	   VIRTIO_CONFIG_S_DRIVER_OK | VIRTIO_CONFIG_S_FEATURES_OK | \
+	   VIRTIO_CONFIG_S_NEEDS_RESET | VIRTIO_CONFIG_S_FAILED))
+
 /*
  * Virtio feature bits VIRTIO_TRANSPORT_F_START through
  * VIRTIO_TRANSPORT_F_END are reserved for the transport
-- 
2.51.1