Re: [PATCH v2 6/7] 9p/trans_virtio: support larger msize values

The virtio transport supports by default a 9p 'msize' of up to
approximately 500 kB. This patch adds support for larger 'msize'
values by resizing the amount of scatter/gather lists if required.

Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
---
 net/9p/trans_virtio.c | 57 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 0db8de84bd51..4cb75f45aa15 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c

@@ -200,6 +200,31 @@ static struct virtqueue_sg *vq_sg_alloc(unsigned int

nsgl) return vq_sg;
 }

+/**
+ * vq_sg_resize - resize passed virtqueue scatter/gather lists to the
passed + * amount of lists
+ * @_vq_sg: scatter/gather lists to be resized
+ * @nsgl: new amount of scatter/gather lists
+ */
+static int vq_sg_resize(struct virtqueue_sg **_vq_sg, unsigned int nsgl)
+{
+	struct virtqueue_sg *vq_sg;
+
+	BUG_ON(!_vq_sg || !nsgl);
+	vq_sg = *_vq_sg;
+	if (vq_sg->nsgl == nsgl)
+		return 0;
+
+	/* lazy resize implementation for now */
+	vq_sg = vq_sg_alloc(nsgl);
+	if (!vq_sg)
+		return -ENOMEM;
+
+	kfree(*_vq_sg);
+	*_vq_sg = vq_sg;
+	return 0;
+}
+
 /**
  * p9_virtio_close - reclaim resources of a channel
  * @client: client instance

@@ -771,6 +796,10 @@ p9_virtio_create(struct p9_client *client, const char

*devname, char *args) struct virtio_chan *chan;
 	int ret = -ENOENT;
 	int found = 0;
+#if !defined(CONFIG_ARCH_NO_SG_CHAIN)
+	size_t npages;
+	size_t nsgl;
+#endif

 	if (devname == NULL)
 		return -EINVAL;

@@ -793,6 +822,34 @@ p9_virtio_create(struct p9_client *client, const char

*devname, char *args) return ret;
 	}

+	/*
+	 * if user supplied an 'msize' option that's larger than what this
+	 * transport supports by default, then try to allocate more sg lists
+	 */
+	if (client->msize > client->trans_maxsize) {
+#ifdef CONFIG_ARCH_NO_SG_CHAIN
+		pr_info("limiting 'msize' to %d because architecture does not "
+			"support chained scatter gather lists\n",
+			client->trans_maxsize);
+#else
+		npages = DIV_ROUND_UP(client->msize, PAGE_SIZE);
+		if (npages > chan->p9_max_pages)
+			npages = chan->p9_max_pages;
+		nsgl = DIV_ROUND_UP(npages, SG_USER_PAGES_PER_LIST);
+		if (nsgl > chan->vq_sg->nsgl) {
+			/*
+			 * if resize fails, no big deal, then just
+			 * continue with default msize instead
+			 */
+			if (!vq_sg_resize(&chan->vq_sg, nsgl)) {
+				client->trans_maxsize =
+					PAGE_SIZE *
+					((nsgl * SG_USER_PAGES_PER_LIST) - 3);
+			}
+		}