[PATCH net-next 4/4] xsk: add helper to get &xdp_desc's DMA and meta pointer in one go

Alexander Lobakin
2025-02-06T18:30:47Z

Currently, when your driver supports XSk Tx metadata and you want to
send an XSk frame, you need to do the following:

* call external xsk_buff_raw_get_dma();
* call inline xsk_buff_get_metadata(), which calls external
xsk_buff_raw_get_data() and then do some inline checks.

This effectively means that the following piece:

addr = pool->unaligned ? xp_unaligned_add_offset_to_addr(addr) : addr;

is done twice per frame, plus you have 2 external calls per frame, plus
this:

meta = pool->addrs + addr - pool->tx_metadata_len;
if (unlikely(!xsk_buff_valid_tx_metadata(meta)))

is always inlined, even if there's no meta or it's invalid.

Add xsk_buff_raw_get_ctx() (xp_raw_get_ctx() to be precise) to do that
in one go. It returns a small structure with 2 fields: DMA address,
filled unconditionally, and metadata pointer, non-NULL only if it's
present and valid. The address correction is performed only once and
you also have only 1 external call per XSk frame, which does all the
calculations and checks outside of your hotpath. You only need to
check `if (ctx.meta)` for the metadata presence.
To not copy any existing code, derive address correction and getting
virtual and DMA address into small helpers. bloat-o-meter reports no
object code changes for the existing functionality.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
---
include/net/xdp_sock_drv.h | 43 +++++++++++++++++++++++++++++++---
include/net/xsk_buff_pool.h | 8 +++++++
net/xdp/xsk_buff_pool.c | 46 +++++++++++++++++++++++++++++++++----
3 files changed, 90 insertions(+), 7 deletions(-)

diff --git a/include/net/xdp_sock_drv.h b/include/net/xdp_sock_drv.h
index 784cd34f5bba..15086dcf51d8 100644
--- a/include/net/xdp_sock_drv.h
+++ b/include/net/xdp_sock_drv.h
@@ -196,6 +196,23 @@ static inline void *xsk_buff_raw_get_data(struct xsk_buff_pool *pool, u64 addr)
return xp_raw_get_data(pool, addr);
}

+/**
+ * xsk_buff_raw_get_ctx - get &xdp_desc context
+ * @pool: XSk buff pool desc address belongs to
+ * @addr: desc address (from userspace)
+ *
+ * Wrapper for xp_raw_get_ctx() to be used in drivers, see its kdoc for
+ * details.
+ *
+ * Return: new &xdp_desc_ctx struct containing desc's DMA address and metadata
+ * pointer, if it is present and valid (initialized to %NULL otherwise).
+ */
+static inline struct xdp_desc_ctx
+xsk_buff_raw_get_ctx(const struct xsk_buff_pool *pool, u64 addr)
+{
+ return xp_raw_get_ctx(pool, addr);
+}
+
#define XDP_TXMD_FLAGS_VALID ( \
XDP_TXMD_FLAGS_TIMESTAMP | \
XDP_TXMD_FLAGS_CHECKSUM | \
@@ -207,20 +224,27 @@ xsk_buff_valid_tx_metadata(const struct xsk_tx_metadata *meta)
return !(meta->flags & ~XDP_TXMD_FLAGS_VALID);
}

-static inline struct xsk_tx_metadata *xsk_buff_get_metadata(struct xsk_buff_pool *pool, u64 addr)
+static inline struct xsk_tx_metadata *
+__xsk_buff_get_metadata(const struct xsk_buff_pool *pool, void *data)
{
struct xsk_tx_metadata *meta;

if (!pool->tx_metadata_len)
return NULL;

- meta = xp_raw_get_data(pool, addr) - pool->tx_metadata_len;
+ meta = data - pool->tx_metadata_len;
if (unlikely(!xsk_buff_valid_tx_metadata(meta)))
return NULL; /* no way to signal the error to the user */

return meta;
}

+static inline struct xsk_tx_metadata *
+xsk_buff_get_metadata(struct xsk_buff_pool *pool, u64 addr)
+{
+ return __xsk_buff_get_metadata(pool, xp_raw_get_data(pool, addr));
+}
+
static inline void xsk_buff_dma_sync_for_cpu(struct xdp_buff *xdp)
{
struct xdp_buff_xsk *xskb = container_of(xdp, struct xdp_buff_xsk, xdp);
@@ -388,12 +412,25 @@ static inline void *xsk_buff_raw_get_data(struct xsk_buff_pool *pool, u64 addr)
return NULL;
}

+static inline struct xdp_desc_ctx
+xsk_buff_raw_get_ctx(const struct xsk_buff_pool *pool, u64 addr)
+{
+ return (struct xdp_desc_ctx){ };
+}
+
static inline bool xsk_buff_valid_tx_metadata(struct xsk_tx_metadata *meta)
{
return false;
}

-static inline struct xsk_tx_metadata *xsk_buff_get_metadata(struct xsk_buff_pool *pool, u64 addr)
+static inline struct xsk_tx_metadata *
+__xsk_buff_get_metadata(const struct xsk_buff_pool *pool, void *data)
+{
+ return NULL;
+}
+
+static inline struct xsk_tx_metadata *
+xsk_buff_get_metadata(struct xsk_buff_pool *pool, u64 addr)
{
return NULL;
}
diff --git a/include/net/xsk_buff_pool.h b/include/net/xsk_buff_pool.h
index 50779406bc2d..1dcd4d71468a 100644
--- a/include/net/xsk_buff_pool.h
+++ b/include/net/xsk_buff_pool.h
@@ -141,6 +141,14 @@ u32 xp_alloc_batch(struct xsk_buff_pool *pool, struct xdp_buff **xdp, u32 max);
bool xp_can_alloc(struct xsk_buff_pool *pool, u32 count);
void *xp_raw_get_data(struct xsk_buff_pool *pool, u64 addr);
dma_addr_t xp_raw_get_dma(struct xsk_buff_pool *pool, u64 addr);
+
+struct xdp_desc_ctx {
+ dma_addr_t dma;
+ struct xsk_tx_metadata *meta;
+};
+
+struct xdp_desc_ctx xp_raw_get_ctx(const struct xsk_buff_pool *pool, u64 addr);
+
static inline dma_addr_t xp_get_dma(struct xdp_buff_xsk *xskb)
{
return xskb->dma;
diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
index 1f7975b49657..c263fb7a68dc 100644
--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -699,18 +699,56 @@ void xp_free(struct xdp_buff_xsk *xskb)
}
EXPORT_SYMBOL(xp_free);

-void *xp_raw_get_data(struct xsk_buff_pool *pool, u64 addr)
+static u64 __xp_raw_get_addr(const struct xsk_buff_pool *pool, u64 addr)
+{
+ return pool->unaligned ? xp_unaligned_add_offset_to_addr(addr) : addr;
+}
+
+static void *__xp_raw_get_data(const struct xsk_buff_pool *pool, u64 addr)
{
- addr = pool->unaligned ? xp_unaligned_add_offset_to_addr(addr) : addr;
return pool->addrs + addr;
}
+
+void *xp_raw_get_data(struct xsk_buff_pool *pool, u64 addr)
+{
+ return __xp_raw_get_data(pool, __xp_raw_get_addr(pool, addr));
+}
EXPORT_SYMBOL(xp_raw_get_data);

-dma_addr_t xp_raw_get_dma(struct xsk_buff_pool *pool, u64 addr)
+static dma_addr_t __xp_raw_get_dma(const struct xsk_buff_pool *pool, u64 addr)
{
- addr = pool->unaligned ? xp_unaligned_add_offset_to_addr(addr) : addr;
return (pool->dma_pages[addr >> PAGE_SHIFT] &
~XSK_NEXT_PG_CONTIG_MASK) +
(addr & ~PAGE_MASK);
}
+
+dma_addr_t xp_raw_get_dma(struct xsk_buff_pool *pool, u64 addr)
+{
+ return __xp_raw_get_dma(pool, __xp_raw_get_addr(pool, addr));
+}
EXPORT_SYMBOL(xp_raw_get_dma);
+
+/**
+ * xp_raw_get_ctx - get &xdp_desc context
+ * @pool: XSk buff pool desc address belongs to
+ * @addr: desc address (from userspace)
+ *
+ * Helper for getting desc's DMA address and metadata pointer, if present.
+ * Saves one call on hotpath, double calculation of the actual address,
+ * and inline checks for metadata presence and sanity.
+ *
+ * Return: new &xdp_desc_ctx struct containing desc's DMA address and metadata
+ * pointer, if it is present and valid (initialized to %NULL otherwise).
+ */
+struct xdp_desc_ctx xp_raw_get_ctx(const struct xsk_buff_pool *pool, u64 addr)
+{
+ struct xdp_desc_ctx ret;
+
+ addr = __xp_raw_get_addr(pool, addr);
+
+ ret.dma = __xp_raw_get_dma(pool, addr);
+ ret.meta = __xsk_buff_get_metadata(pool, __xp_raw_get_data(pool, addr));
+
+ return ret;
+}
+EXPORT_SYMBOL(xp_raw_get_ctx);
--
2.48.1