Re: the XSK buffer pool needs be to reverted

[BjÃrn TÃpel]

On 2020-06-26 09:47, Christoph Hellwig wrote:
> Hi BjÃrn,
>
> you addition of the xsk_buff_pool.c APIs in commit 2b43470add8c
> ("xsk: Introduce AF_XDP buffer allocation API") is unfortunately rather
> broken by making lots of assumptions and poking into dma-direct and
> swiotlb internals that are of no business to outside users and clearly
> marked as such.   I'd be glad to work with your doing something proper
> for pools, but that needs proper APIs and probably live in the dma
> mapping core, but for that you'd actually need to contact the relevant
> maintainers before poking into internals.

Christoph,

Thanks for clarifying that. Let's work on a solution that can reside in
the dma mapping core.

> The commit seems to have a long dove tail of commits depending on it
> despite only being a month old, so maybe you can do the revert for now?

Reverting the whole series sounds a bit too much. Let's focus on the
part that breaks the dma api abstraction. I'm assuming that you're
referring to the

  static bool xp_check_cheap_dma(struct xsk_buff_pool *pool)

function (and related functions called from that)?

> Note that this is somewhat urgent, as various of the APIs that the code
> is abusing are slated to go away for Linux 5.9, so this addition comes
> at a really bad time.

Understood. Wdyt about something in the lines of the diff below? It's
build tested only, but removes all non-dma API usage ("poking
internals"). Would that be a way forward, and then as a next step work
on a solution that would give similar benefits, but something that would
live in the dma mapping core?

diff --git a/include/net/xsk_buff_pool.h b/include/net/xsk_buff_pool.h
index a4ff226505c9..003b172ce0d2 100644
--- a/include/net/xsk_buff_pool.h
+++ b/include/net/xsk_buff_pool.h
@@ -40,7 +40,6 @@ struct xsk_buff_pool {
 	u32 headroom;
 	u32 chunk_size;
 	u32 frame_len;
-	bool cheap_dma;
 	bool unaligned;
 	void *addrs;
 	struct device *dev;
@@ -80,9 +79,6 @@ static inline dma_addr_t xp_get_frame_dma(struct 
xdp_buff_xsk *xskb)
 void xp_dma_sync_for_cpu_slow(struct xdp_buff_xsk *xskb);
 static inline void xp_dma_sync_for_cpu(struct xdp_buff_xsk *xskb)
 {
-	if (xskb->pool->cheap_dma)
-		return;
-
 	xp_dma_sync_for_cpu_slow(xskb);
 }

@@ -91,9 +87,6 @@ void xp_dma_sync_for_device_slow(struct xsk_buff_pool 
*pool, dma_addr_t dma,
 static inline void xp_dma_sync_for_device(struct xsk_buff_pool *pool,
 					  dma_addr_t dma, size_t size)
 {
-	if (pool->cheap_dma)
-		return;
-
 	xp_dma_sync_for_device_slow(pool, dma, size);
 }

diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
index 540ed75e4482..5714f3711381 100644
--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -2,9 +2,6 @@

 #include <net/xsk_buff_pool.h>
 #include <net/xdp_sock.h>
-#include <linux/dma-direct.h>
-#include <linux/dma-noncoherent.h>
-#include <linux/swiotlb.h>

 #include "xsk_queue.h"

@@ -55,7 +52,6 @@ struct xsk_buff_pool *xp_create(struct page **pages, 
u32 nr_pages, u32 chunks,
 	pool->free_heads_cnt = chunks;
 	pool->headroom = headroom;
 	pool->chunk_size = chunk_size;
-	pool->cheap_dma = true;
 	pool->unaligned = unaligned;
 	pool->frame_len = chunk_size - headroom - XDP_PACKET_HEADROOM;
 	INIT_LIST_HEAD(&pool->free_list);
@@ -125,48 +121,6 @@ static void xp_check_dma_contiguity(struct 
xsk_buff_pool *pool)
 	}
 }

-static bool __maybe_unused xp_check_swiotlb_dma(struct xsk_buff_pool *pool)
-{
-#if defined(CONFIG_SWIOTLB)
-	phys_addr_t paddr;
-	u32 i;
-
-	for (i = 0; i < pool->dma_pages_cnt; i++) {
-		paddr = dma_to_phys(pool->dev, pool->dma_pages[i]);
-		if (is_swiotlb_buffer(paddr))
-			return false;
-	}
-#endif
-	return true;
-}
-
-static bool xp_check_cheap_dma(struct xsk_buff_pool *pool)
-{
-#if defined(CONFIG_HAS_DMA)
-	const struct dma_map_ops *ops = get_dma_ops(pool->dev);
-
-	if (ops) {
-		return !ops->sync_single_for_cpu &&
-			!ops->sync_single_for_device;
-	}
-
-	if (!dma_is_direct(ops))
-		return false;
-
-	if (!xp_check_swiotlb_dma(pool))
-		return false;
-
-	if (!dev_is_dma_coherent(pool->dev)) {
-#if defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU) ||		\
-	defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL) ||	\
-	defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE)
-		return false;
-#endif
-	}
-#endif
-	return true;
-}
-
 int xp_dma_map(struct xsk_buff_pool *pool, struct device *dev,
 	       unsigned long attrs, struct page **pages, u32 nr_pages)
 {
@@ -195,7 +149,6 @@ int xp_dma_map(struct xsk_buff_pool *pool, struct 
device *dev,
 		xp_check_dma_contiguity(pool);

 	pool->dev = dev;
-	pool->cheap_dma = xp_check_cheap_dma(pool);
 	return 0;
 }
 EXPORT_SYMBOL(xp_dma_map);
@@ -280,11 +233,9 @@ struct xdp_buff *xp_alloc(struct xsk_buff_pool *pool)
 	xskb->xdp.data = xskb->xdp.data_hard_start + XDP_PACKET_HEADROOM;
 	xskb->xdp.data_meta = xskb->xdp.data;

-	if (!pool->cheap_dma) {
-		dma_sync_single_range_for_device(pool->dev, xskb->dma, 0,
-						 pool->frame_len,
-						 DMA_BIDIRECTIONAL);
-	}
+	dma_sync_single_range_for_device(pool->dev, xskb->dma, 0,
+					 pool->frame_len,
+					 DMA_BIDIRECTIONAL);
 	return &xskb->xdp;
 }
 EXPORT_SYMBOL(xp_alloc);