[RFC PATCH 1/4] common: add UAPI for SQ/CQ doorbell polling

From: Yufeng Wang <[email protected]>

Define VIRTIO_F_SQCQ_POLL (bit 42) and the associated SQ/CQ doorbell
structures for split virtqueue polling mode.

  - include/uapi/linux/virtio_config.h: feature bit 42
  - include/uapi/linux/virtio_ring.h: struct vring_sq, struct vring_cq
    (128-byte aligned for cache-line isolation on ARM Neoverse N2/V2)
  - include/uapi/linux/virtio_pci.h: sqe/cqe_ring PCI config registers
    at offsets 64-76

Signed-off-by: Yufeng Wang <[email protected]>
---
 include/uapi/linux/virtio_config.h |  8 +++++++-
 include/uapi/linux/virtio_pci.h    |  9 +++++++++
 include/uapi/linux/virtio_ring.h   | 25 +++++++++++++++++++++++++
 3 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/virtio_config.h b/include/uapi/linux/virtio_config.h
index 2445f365bce7..dbda23e44b62 100644
--- a/include/uapi/linux/virtio_config.h
+++ b/include/uapi/linux/virtio_config.h
@@ -52,7 +52,7 @@
  * rest are per-device feature bits.
  */
 #define VIRTIO_TRANSPORT_F_START	28
-#define VIRTIO_TRANSPORT_F_END		42
+#define VIRTIO_TRANSPORT_F_END		43
 
 #ifndef VIRTIO_CONFIG_NO_LEGACY
 /* Do we get callbacks when the ring is completely used, even if we've
@@ -120,4 +120,10 @@
  */
 #define VIRTIO_F_ADMIN_VQ		41
 
+/*
+ * This feature indicates that both driver and device support
+ * SQ/CQ polling mode to replace interrupt-based notifications.
+ */
+#define VIRTIO_F_SQCQ_POLL		42
+
 #endif /* _UAPI_LINUX_VIRTIO_CONFIG_H */
diff --git a/include/uapi/linux/virtio_pci.h b/include/uapi/linux/virtio_pci.h
index e732e3456e27..f5058d1b4b16 100644
--- a/include/uapi/linux/virtio_pci.h
+++ b/include/uapi/linux/virtio_pci.h
@@ -193,6 +193,11 @@ struct virtio_pci_modern_common_cfg {
 
 	__le16 admin_queue_index;	/* read-only */
 	__le16 admin_queue_num;		/* read-only */
+
+	__le32 sqe_ring_lo;		/* read-write: SQ DMA addr low */
+	__le32 sqe_ring_hi;		/* read-write: SQ DMA addr high */
+	__le32 cqe_ring_lo;		/* read-write: CQ DMA addr low */
+	__le32 cqe_ring_hi;		/* read-write: CQ DMA addr high */
 };
 
 /* Fields in VIRTIO_PCI_CAP_PCI_CFG: */
@@ -235,6 +240,10 @@ 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_SQE_LO	64
+#define VIRTIO_PCI_COMMON_SQE_HI	68
+#define VIRTIO_PCI_COMMON_CQE_LO	72
+#define VIRTIO_PCI_COMMON_CQE_HI	76
 
 #endif /* VIRTIO_PCI_NO_MODERN */
 
diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
index 3c478582a3c2..d1f2b9584d7c 100644
--- a/include/uapi/linux/virtio_ring.h
+++ b/include/uapi/linux/virtio_ring.h
@@ -131,6 +131,31 @@ struct vring_used {
 	vring_used_elem_t ring[];
 };
 
+/*
+ * SQ/CQ doorbell structures for polling mode (VIRTIO_F_SQCQ_POLL).
+ * These are allocated separately from the vring and their DMA addresses
+ * are communicated via PCI config space registers.
+ *
+ * Ownership:
+ *   SQ: Guest writes idx, Device reads it.
+ *   CQ: Device writes idx, Guest reads it.
+ *
+ * Flags (8-bit):
+ *   Set to 1 before sleeping, cleared to 0 after waking up.
+ *   Direct assignment — no read-modify-write.
+ */
+struct vring_sq {
+	__virtio64 idx;			/* Producer position (updated by guest) */
+	__u8 sq_need_wakeup;		/* 1 = device should be woken on next kick */
+	__u8 reserved[55];		/* Reserved for future use */
+} __attribute__((aligned(128)));
+
+struct vring_cq {
+	__virtio64 idx;			/* Producer position (updated by device) */
+	__u8 cq_need_wakeup;		/* 1 = guest should be woken on completion */
+	__u8 reserved[55];		/* Reserved for future use */
+} __attribute__((aligned(128)));
+
 /*
  * The ring element addresses are passed between components with different
  * alignments assumptions. Thus, we might need to decrease the compiler-selected
-- 
2.34.1