[RFC PATCH] nvme: enable PCI P2PDMA support for RDMA transport

[Shivaji Kant]

Enable BLK_FEAT_PCI_P2PDMA on the NVMe when the underlying
RDMA controller supports it.

blk_stack_limits() currently filters out this feature bit because it is
absent from BLK_FEAT_INHERIT_MASK. Manually re-assert the capability
in nvme_update_ns_info() after the stacking operation.

Hardware reachability remains enforced by late-stage distance checks
during DMA mapping.

Suggested-by: Pranjal Shrivastava <praan at google.com>
Signed-off-by: Shivaji Kant <shivajikant at google.com>
---
 drivers/nvme/host/core.c |  9 +++++++++
 drivers/nvme/host/rdma.c | 15 +++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index a2da54f974fa..0d7b0f286895 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -2205,6 +2205,15 @@ static int nvme_update_ns_info_generic(struct nvme_ns *ns,
 	nvme_set_ctrl_limits(ns->ctrl, &lim, false);
 
 	memflags = blk_mq_freeze_queue(ns->disk->queue);
+
+	/*
+	 * Explicitly check for P2PDMA support as BLK_FEAT_PCI_P2PDMA
+	 * is filtered out by queue_limits_stack_bdev().
+	 */
+	if (ns->ctrl->ops->supports_pci_p2pdma &&
+	   ns->ctrl->ops->supports_pci_p2pdma(ns->ctrl))
+		lim.features |= BLK_FEAT_PCI_P2PDMA;
+
 	ret = queue_limits_commit_update(ns->disk->queue, &lim);
 	set_disk_ro(ns->disk, nvme_ns_is_readonly(ns, info));
 	blk_mq_unfreeze_queue(ns->disk->queue, memflags);
diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c
index 35c0822edb2d..3ce6f3e476b0 100644
--- a/drivers/nvme/host/rdma.c
+++ b/drivers/nvme/host/rdma.c
@@ -2189,6 +2189,20 @@ static void nvme_rdma_reset_ctrl_work(struct work_struct *work)
 	nvme_rdma_reconnect_or_remove(ctrl, ret);
 }
 
+static bool nvme_rdma_supports_pci_p2pdma(struct nvme_ctrl *ctrl)
+{
+	struct nvme_rdma_ctrl *r_ctrl = to_rdma_ctrl(ctrl);
+	bool supported = false;
+
+	if (r_ctrl && r_ctrl->device)
+		supported = ib_dma_pci_p2p_dma_supported(r_ctrl->device->dev);
+
+	dev_dbg(ctrl->device, "PCI P2PDMA support result: %s\n",
+			supported ? "PASSED" : "FAILED (HW/Driver restriction)");
+
+	return supported;
+}
+
 static const struct nvme_ctrl_ops nvme_rdma_ctrl_ops = {
 	.name			= "rdma",
 	.module			= THIS_MODULE,
@@ -2203,6 +2217,7 @@ static const struct nvme_ctrl_ops nvme_rdma_ctrl_ops = {
 	.get_address		= nvmf_get_address,
 	.stop_ctrl		= nvme_rdma_stop_ctrl,
 	.get_virt_boundary	= nvme_get_virt_boundary,
+	.supports_pci_p2pdma	= nvme_rdma_supports_pci_p2pdma,
 };
 
 /*
-- 
2.53.0.1213.gd9a14994de-goog