Re: [RFC PATCH 1/6] virtio/console: Add splice_write support

On (Tue) 24 Jul 2012 [11:37:07], Yoshihiro YUNOMAE wrote:

From: Masami Hiramatsu <redacted>

Enable to use splice_write from pipe to virtio-console port.
This steals pages from pipe and directly send it to host.

Note that this may accelerate only the guest to host path.

Signed-off-by: Masami Hiramatsu <redacted>
Cc: Amit Shah <redacted>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

+/* Faster zero-copy write by splicing */
+static ssize_t port_fops_splice_write(struct pipe_inode_info *pipe,
+				      struct file *filp, loff_t *ppos,
+				      size_t len, unsigned int flags)
+{
+	struct port *port = filp->private_data;
+	struct sg_list sgl;
+	ssize_t ret;
+	struct splice_desc sd = {
+		.total_len = len,
+		.flags = flags,
+		.pos = *ppos,
+		.u.data = &sgl,
+	};
+
+	sgl.n = 0;
+	sgl.len = 0;
+	sgl.sg = kmalloc(sizeof(struct scatterlist) * MAX_SPLICE_PAGES,
+			 GFP_ATOMIC);

Do you expect this function to be called from interrupt context?

+	if (unlikely(!sgl.sg))
+		return -ENOMEM;
+
+	sg_init_table(sgl.sg, MAX_SPLICE_PAGES);
+	ret = __splice_from_pipe(pipe, &sd, pipe_to_sg);
+	if (likely(ret > 0))
+		ret = send_pages(port, sgl.sg, sgl.n, sgl.len, true);
+
+	return ret;
+}

		Amit