linux-kernel - Re: [RFC PATCH 1/6] virtio/console: Add splice

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Date:	Thu, 9 Aug 2012 14:30:15 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>
Cc:	linux-kernel@...r.kernel.org,
	Anthony Liguori <anthony@...emonkey.ws>,
	Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...64.org>,
	"Franch Ch. Eigler" <fche@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	Ingo Molnar <mingo@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	virtualization@...ts.linux-foundation.org, qemu-devel@...gnu.org,
	yrl.pp-manager.tt@...achi.com,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Subject: 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 <masami.hiramatsu.pt@...achi.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@...achi.com>
> Cc: Amit Shah <amit.shah@...hat.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.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
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