linux-kernel - Re: [Qemu-devel] [RFC PATCH 1/2] ivring: Add a ring-buffer driver on IVShmem

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Date:	Wed, 06 Jun 2012 07:03:06 +0800
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Borislav Petkov <bp@...64.org>
CC:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>,
	Ohad Ben-Cohen <ohad@...ery.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	qemu-devel@...gnu.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	Borislav Petkov <borislav.petkov@....com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	MyungJoo Ham <myungjoo.ham@...sung.com>,
	systemtap@...rceware.org, Joerg Roedel <joerg.roedel@....com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	yrl.pp-manager.tt@...achi.com, Cam Macdonell <cam@...ualberta.ca>,
	Akihiro Nagai <akihiro.nagai.hw@...achi.com>
Subject: Re: [Qemu-devel] [RFC PATCH 1/2] ivring: Add a ring-buffer driver
 on IVShmem

On 06/05/2012 09:10 PM, Borislav Petkov wrote:
> On Tue, Jun 05, 2012 at 10:01:17PM +0900, Yoshihiro YUNOMAE wrote:
>> This patch adds a ring-buffer driver for IVShmem device, a virtual RAM device in
>> QEMU. This driver can be used as a ring-buffer for kernel logging or tracing of
>> a guest OS by recording kernel programing or SystemTap.
>>
>> This ring-buffer driver is implemented very simple. First 4kB of shared memory
>> region is control structure of a ring-buffer. In this region, some values for
>> managing the ring-buffer is stored such as bits and mask of whole memory size,
>> writing position, threshold value for notification to a reader on a host OS.
>> This region is used by the reader to know writing position. Then, "total
>> memory size - 4kB" equals to usable memory region for recording data.
>> This ring-buffer driver records any data from start to end of the writable
>> memory region.
>>
>> When writing size exceeds a threshold value, this driver can notify a reader
>> to read data by using writel(). As this later patch, reader does not have any
>> function for receiving the notification. This notification feature will be used
>> near the future.
>>
>> As a writer records data in this ring-buffer, spinlock function is used to
>> avoid competing by some writers in multi CPU environment. Not to use spinlock,
>> lockless ring-buffer like as ftrace and one ring-buffer one CPU will be
>> implemented near the future.
>
> Yet another ring buffer?
>
> We already have an ftrace and perf ring buffer, can't you use one of those?

Not to mention virtio :-)

Why not just make a virtio device for this kind of thing?

Regards,

Anthony Liguori

>

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