net: xsk: add a simple buffer reuse queue

XSK UMEM is strongly single producer single consumer so reuse of
frames is challenging.  Add a simple "stash" of FILL packets to
reuse for drivers to optionally make use of.  This is useful
when driver has to free (ndo_stop) or resize a ring with an active
AF_XDP ZC socket.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

net/xdp/xsk_queue.c

@@ -3,7 +3,9 @@
  * Copyright(c) 2018 Intel Corporation.
  */
 
+#include <linux/log2.h>
 #include <linux/slab.h>
+#include <linux/overflow.h>
 
 #include "xsk_queue.h"
 
@@ -62,3 +64,56 @@ void xskq_destroy(struct xsk_queue *q)
 	page_frag_free(q->ring);
 	kfree(q);
 }
+
+struct xdp_umem_fq_reuse *xsk_reuseq_prepare(u32 nentries)
+{
+	struct xdp_umem_fq_reuse *newq;
+
+	/* Check for overflow */
+	if (nentries > (u32)roundup_pow_of_two(nentries))
+		return NULL;
+	nentries = roundup_pow_of_two(nentries);
+
+	newq = kvmalloc(struct_size(newq, handles, nentries), GFP_KERNEL);
+	if (!newq)
+		return NULL;
+	memset(newq, 0, offsetof(typeof(*newq), handles));
+
+	newq->nentries = nentries;
+	return newq;
+}
+EXPORT_SYMBOL_GPL(xsk_reuseq_prepare);
+
+struct xdp_umem_fq_reuse *xsk_reuseq_swap(struct xdp_umem *umem,
+					  struct xdp_umem_fq_reuse *newq)
+{
+	struct xdp_umem_fq_reuse *oldq = umem->fq_reuse;
+
+	if (!oldq) {
+		umem->fq_reuse = newq;
+		return NULL;
+	}
+
+	if (newq->nentries < oldq->length)
+		return newq;
+
+	memcpy(newq->handles, oldq->handles,
+	       array_size(oldq->length, sizeof(u64)));
+	newq->length = oldq->length;
+
+	umem->fq_reuse = newq;
+	return oldq;
+}
+EXPORT_SYMBOL_GPL(xsk_reuseq_swap);
+
+void xsk_reuseq_free(struct xdp_umem_fq_reuse *rq)
+{
+	kvfree(rq);
+}
+EXPORT_SYMBOL_GPL(xsk_reuseq_free);
+
+void xsk_reuseq_destroy(struct xdp_umem *umem)
+{
+	xsk_reuseq_free(umem->fq_reuse);
+	umem->fq_reuse = NULL;
+}