Re: [Qemu-devel] [RFC PATCH 0/6] virtio-trace: Support virtio-trace

[Masami Hiramatsu]

(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: address@hidden