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

[Amit Shah <amit.shah@redhat.com>]

On (Tue) 24 Jul 2012 [11:37:07], Yoshihiro YUNOMAE wrote:
> From: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> 
> 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 <masami.hiramatsu.pt@hitachi.com>
> Cc: Amit Shah <amit.shah@redhat.com>
> 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