[RFC PATCH 04/12] virtio_pci: read the device memory buffer shared memory id

[Alexander Graf <graf@amazon.com>]

In preparation to support VIRTIO_F_DMB, read the shared memory id of
the Device Memory Buffer. A device that negotiates VIRTIO_F_DMB reports
it in a read-only le16 at the end of the common configuration
structure, and it names the VIRTIO_PCI_CAP_SHARED_MEMORY_CFG capability
that virtio_pci_find_shm_cap() has to find to map the region.

Add the dmb_shm_id field, VIRTIO_PCI_COMMON_DMB_SHM_ID for the offset
check_offsets() asserts it against, and vp_modern_get_dmb_shm_id() to
read the register. vp_modern_probe() capped the common cfg mapping at
the end of admin_queue_num, exactly where the new field starts, so
extend it to the end of dmb_shm_id. vp_modern_map_capability() takes
that as an upper bound, so a device with a shorter common cfg maps what
it has and mdev->common_len records how much.

The offset and VIRTIO_F_DMB are provisional until the virtio Technical
Committee allocates them.

Link: https://lore.kernel.org/virtio-comment/20260804161202.38619-1-graf@amazon.com/
Assisted-by: Kiro:claude-opus-5 checkpatch sparse
Signed-off-by: Alexander Graf <graf@amazon.com>
---
 drivers/virtio/virtio_pci_modern_dev.c | 23 ++++++++++++++++++++++-
 include/linux/virtio_pci_modern.h      |  1 +
 include/uapi/linux/virtio_pci.h        | 10 ++++++++++
 3 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_pci_modern_dev.c b/drivers/virtio/virtio_pci_modern_dev.c
index 413a8c353463..7fbfcb305a62 100644
--- a/drivers/virtio/virtio_pci_modern_dev.c
+++ b/drivers/virtio/virtio_pci_modern_dev.c
@@ -211,6 +211,8 @@ static inline void check_offsets(void)
 		     offsetof(struct virtio_pci_modern_common_cfg, admin_queue_index));
 	BUILD_BUG_ON(VIRTIO_PCI_COMMON_ADM_Q_NUM !=
 		     offsetof(struct virtio_pci_modern_common_cfg, admin_queue_num));
+	BUILD_BUG_ON(VIRTIO_PCI_COMMON_DMB_SHM_ID !=
+		     offsetof(struct virtio_pci_modern_common_cfg, dmb_shm_id));
 }
 
 /*
@@ -300,7 +302,7 @@ int vp_modern_probe(struct virtio_pci_modern_device *mdev)
 	mdev->common = vp_modern_map_capability(mdev, common,
 			      sizeof(struct virtio_pci_common_cfg), 4, 0,
 			      offsetofend(struct virtio_pci_modern_common_cfg,
-					  admin_queue_num),
+					  dmb_shm_id),
 			      &mdev->common_len, NULL);
 	if (!mdev->common)
 		goto err_map_common;
@@ -752,6 +754,25 @@ u16 vp_modern_avq_index(struct virtio_pci_modern_device *mdev)
 }
 EXPORT_SYMBOL_GPL(vp_modern_avq_index);
 
+/*
+ * vp_modern_get_dmb_shm_id - read the Device Memory Buffer shared memory id
+ * @mdev: the modern virtio-pci device
+ *
+ * The value identifies the VIRTIO_PCI_CAP_SHARED_MEMORY_CFG capability that
+ * describes the Device Memory Buffer region.  Only valid once VIRTIO_F_DMB
+ * has been negotiated.
+ *
+ * Returns the shared memory id.
+ */
+u16 vp_modern_get_dmb_shm_id(struct virtio_pci_modern_device *mdev)
+{
+	struct virtio_pci_modern_common_cfg __iomem *cfg;
+
+	cfg = (struct virtio_pci_modern_common_cfg __iomem *)mdev->common;
+	return vp_ioread16(&cfg->dmb_shm_id);
+}
+EXPORT_SYMBOL_GPL(vp_modern_get_dmb_shm_id);
+
 MODULE_VERSION("0.1");
 MODULE_DESCRIPTION("Modern Virtio PCI Device");
 MODULE_AUTHOR("Jason Wang <jasowang@redhat.com>");
diff --git a/include/linux/virtio_pci_modern.h b/include/linux/virtio_pci_modern.h
index 9a3f2fc53bd6..c396f8e6055b 100644
--- a/include/linux/virtio_pci_modern.h
+++ b/include/linux/virtio_pci_modern.h
@@ -162,4 +162,5 @@ int vp_modern_get_queue_reset(struct virtio_pci_modern_device *mdev, u16 index);
 void vp_modern_set_queue_reset(struct virtio_pci_modern_device *mdev, u16 index);
 u16 vp_modern_avq_num(struct virtio_pci_modern_device *mdev);
 u16 vp_modern_avq_index(struct virtio_pci_modern_device *mdev);
+u16 vp_modern_get_dmb_shm_id(struct virtio_pci_modern_device *mdev);
 #endif
diff --git a/include/uapi/linux/virtio_pci.h b/include/uapi/linux/virtio_pci.h
index e732e3456e27..052a1047f48f 100644
--- a/include/uapi/linux/virtio_pci.h
+++ b/include/uapi/linux/virtio_pci.h
@@ -193,6 +193,15 @@ struct virtio_pci_modern_common_cfg {
 
 	__le16 admin_queue_index;	/* read-only */
 	__le16 admin_queue_num;		/* read-only */
+
+	/*
+	 * Reports the shmid of the Device Memory Buffer region.  The feature
+	 * is a specification proposal before the virtio Technical Committee,
+	 * and both this offset and the VIRTIO_F_DMB feature bit are
+	 * provisional: neither has been allocated, so a device must not rely
+	 * on either until they have been.
+	 */
+	__le16 dmb_shm_id;		/* read-only */
 };
 
 /* Fields in VIRTIO_PCI_CAP_PCI_CFG: */
@@ -235,6 +244,7 @@ struct virtio_pci_cfg_cap {
 #define VIRTIO_PCI_COMMON_Q_RESET	58
 #define VIRTIO_PCI_COMMON_ADM_Q_IDX	60
 #define VIRTIO_PCI_COMMON_ADM_Q_NUM	62
+#define VIRTIO_PCI_COMMON_DMB_SHM_ID	64
 
 #endif /* VIRTIO_PCI_NO_MODERN */