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

From: Masami Hiramatsu
Subject: Re: [Qemu-devel] [RFC PATCH 1/2] ivring: Add a ring-buffer driver on IVShmem
Date: Wed, 06 Jun 2012 23:44:28 +0900
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:12.0) Gecko/20120428 Thunderbird/12.0.1

(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,

Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: address@hidden

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