Date:	Wed, 06 Jun 2012 23:44:28 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Anthony Liguori <anthony@...emonkey.ws>,
	Borislav Petkov <bp@...64.org>,
	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>,
	Ohad Ben-Cohen <ohad@...ery.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	qemu-devel@...gnu.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>,
	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

(2012/06/06 8:22), Greg Kroah-Hartman wrote:
> On Wed, Jun 06, 2012 at 07:03:06AM +0800, Anthony Liguori wrote:
>> 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?
> 
> Yeah, that's exactly what I was thinking, why reinvent things again?

Agreed. Actually, we think this is just a concept prototype.
Because of many restrictions of this device, especially for
scalability (which Yoshihiro will give a talk in LinuxCon Japan),
we are considering to move onto a virtio-based shmem device.

Afaics, it seems possible to use it virtio-ballon like way to pass
actual pages of the guest ring buffer to host. Then the reader
can read the pages directly from qemu.

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com
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