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

From: Yoshihiro YUNOMAE
Subject: Re: [Qemu-devel] [RFC PATCH 1/2] ivring: Add a ring-buffer driver on IVShmem
Date: Tue, 05 Jun 2012 23:13:05 +0900
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:12.0) Gecko/20120428 Thunderbird/12.0.1

(2012/06/05 22:10), 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?

Yes, unfortunately...

We already have an ftrace and perf ring buffer, can't you use one of those?

No, because those do not support to allocate buffer
from PCI memory device, nor pass the control structure
over it.

However, indeed, we understand what you would like to say.

This series is just RFC and we'd like to ask who is
interested in the guest tracing and how it should be

 - no more ring buffer. enhance perf/ftrace ring buffer to
   enable allocating buffers on shared memory.

Other comments are welcome.

Thank you,

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

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