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 <kbusch@kernel.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250227103707.2640014-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index ff648c6839c15446ab845ea3fbbb57713a665495..e8f52d8341fba82da17c0a2c01c3aa1d320964c9 100644 (file)
--- a/drivers/block/ublk_drv.c
+++ b/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;
 

diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h
index a8bc98bb69fce395e88986c5e013b1934830795e..8093acdeaa1141bf3b1ca2ffcc8d6ac4bc0a59d7 100644 (file)
--- a/include/uapi/linux/ublk_cmd.h
+++ b/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