[PATCH net-next v2 16/18] xsk: add helper to get &xdp_desc's DMA and meta pointer in one go

Alexander Lobakin
2024-10-15T14:55:20Z

Currently, when you send an XSk frame without metadata, 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, valid only if it's
present. 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.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
---
include/net/xdp_sock_drv.h | 23 +++++++++++++++++++++
include/net/xsk_buff_pool.h | 8 ++++++++
net/xdp/xsk_buff_pool.c | 40 +++++++++++++++++++++++++++++++++++++
3 files changed, 71 insertions(+)

diff --git a/include/net/xdp_sock_drv.h b/include/net/xdp_sock_drv.h
index 6aae95b83645..324a4bb04431 100644
--- a/include/net/xdp_sock_drv.h
+++ b/include/net/xdp_sock_drv.h
@@ -205,6 +205,23 @@ 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 | \
@@ -402,6 +419,12 @@ 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;
diff --git a/include/net/xsk_buff_pool.h b/include/net/xsk_buff_pool.h
index 3832997cc605..6c540696a299 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 ae71da7d2cd6..02c42caec9f4 100644
--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -715,3 +715,43 @@ dma_addr_t xp_raw_get_dma(struct xsk_buff_pool *pool, u64 addr)
(addr & ~PAGE_MASK);
}
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.
+ * Please use xsk_buff_raw_get_ctx() in drivers instead.
+ *
+ * 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 xsk_tx_metadata *meta;
+ struct xdp_desc_ctx ret;
+
+ addr = pool->unaligned ? xp_unaligned_add_offset_to_addr(addr) : addr;
+ ret = (typeof(ret)){
+ /* Same logic as in xp_raw_get_dma() */
+ .dma = (pool->dma_pages[addr >> PAGE_SHIFT] &
+ ~XSK_NEXT_PG_CONTIG_MASK) + (addr & ~PAGE_MASK),
+ };
+
+ if (!pool->tx_metadata_len)
+ goto out;
+
+ /* Same logic as in xp_raw_get_data() + xsk_buff_get_metadata() */
+ meta = pool->addrs + addr - pool->tx_metadata_len;
+ if (unlikely(!xsk_buff_valid_tx_metadata(meta)))
+ goto out;
+
+ ret.meta = meta;
+
+out:
+ return ret;
+}
+EXPORT_SYMBOL(xp_raw_get_ctx);
--
2.46.2