ublk: add DMA alignment limit

The in-tree ublk driver doesn't need DMA alignment limit because there
is one data copy between request pages and the userspace buffer.

However, ublk is going to support zero copy, then DMA alignment limit
is required, because same IO buffer is forwarded to backend which may
have specific buffer DMA alignment limit, so the limit has to be exposed
from the frontend driver to client application.

Cc: Keith Busch <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Ming Lei

drivers/block/ublk_drv.c

@@ -70,7 +70,8 @@
 /* All UBLK_PARAM_TYPE_* should be included here */
 #define UBLK_PARAM_TYPE_ALL                                \
 	(UBLK_PARAM_TYPE_BASIC | UBLK_PARAM_TYPE_DISCARD | \
-	 UBLK_PARAM_TYPE_DEVT | UBLK_PARAM_TYPE_ZONED)
+	 UBLK_PARAM_TYPE_DEVT | UBLK_PARAM_TYPE_ZONED |    \
+	 UBLK_PARAM_TYPE_DMA_ALIGN)
 
 struct ublk_rq_data {
 	struct llist_node node;
@@ -568,6 +569,16 @@ static int ublk_validate_params(const struct ublk_device *ub)
 	else if (ublk_dev_is_zoned(ub))
 		return -EINVAL;
 
+	if (ub->params.types & UBLK_PARAM_TYPE_DMA_ALIGN) {
+		const struct ublk_param_dma_align *p = &ub->params.dma;
+
+		if (p->alignment >= PAGE_SIZE)
+			return -EINVAL;
+
+		if (!is_power_of_2(p->alignment + 1))
+			return -EINVAL;
+	}
+
 	return 0;
 }
 
@@ -2300,6 +2311,9 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub, struct io_uring_cmd *cmd)
 	if (ub->params.basic.attrs & UBLK_ATTR_ROTATIONAL)
 		lim.features |= BLK_FEAT_ROTATIONAL;
 
+	if (ub->params.types & UBLK_PARAM_TYPE_DMA_ALIGN)
+		lim.dma_alignment = ub->params.dma.alignment;
+
 	if (wait_for_completion_interruptible(&ub->completion) != 0)
 		return -EINTR;
 

include/uapi/linux/ublk_cmd.h

@@ -401,6 +401,11 @@ struct ublk_param_zoned {
 	__u8	reserved[20];
 };
 
+struct ublk_param_dma_align {
+	__u32	alignment;
+	__u8	pad[4];
+};
+
 struct ublk_params {
 	/*
 	 * Total length of parameters, userspace has to set 'len' for both
@@ -413,12 +418,14 @@ struct ublk_params {
 #define UBLK_PARAM_TYPE_DISCARD         (1 << 1)
 #define UBLK_PARAM_TYPE_DEVT            (1 << 2)
 #define UBLK_PARAM_TYPE_ZONED           (1 << 3)
+#define UBLK_PARAM_TYPE_DMA_ALIGN       (1 << 4)
 	__u32	types;			/* types of parameter included */
 
 	struct ublk_param_basic		basic;
 	struct ublk_param_discard	discard;
 	struct ublk_param_devt		devt;
 	struct ublk_param_zoned	zoned;
+	struct ublk_param_dma_align	dma;
 };
 
 #endif