linux-kernel - Re: [RFC PATCH 0/6] virtio-trace: Support virtio-trace

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Date:	Tue, 24 Jul 2012 12:27:27 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>
Cc:	linux-kernel@...r.kernel.org, Amit Shah <amit.shah@...hat.com>,
	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.apana.org.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
Subject: Re: [RFC PATCH 0/6] virtio-trace: Support virtio-trace

(2012/07/24 11:36), Yoshihiro YUNOMAE wrote:
> Therefore, we propose a new system "virtio-trace", which uses enhanced
> virtio-serial and existing ring-buffer of ftrace, for collecting guest kernel
> tracing data. In this system, there are 5 main components:
>  (1) Ring-buffer of ftrace in a guest
>      - When trace agent reads ring-buffer, a page is removed from ring-buffer.
>  (2) Trace agent in the guest
>      - Splice the page of ring-buffer to read_pipe using splice() without
>        memory copying. Then, the page is spliced from write_pipe to virtio
>        without memory copying.
>  (3) Virtio-console driver in the guest
>      - Pass the page to virtio-ring
>  (4) Virtio-serial bus in QEMU
>      - Copy the page to kernel pipe
>  (5) Reader in the host
>      - Read guest tracing data via FIFO(named pipe)

So, this is our answer for the argued points in previous thread.
This virtio-serial and ftrace enhancements doesn't introduce new
"ringbuffer" in the kernel, and just use virtio's ringbuffer.
Also, using splice gives us a great advantage in the performance
because of copy-less trace-data transfer.

Actually, one copy should occur in the host (to write it into the pipe),
because removing physical pages of the guest is hard to track and may
involve a TLB flush per page, even if it is done in background.

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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