Re: [Qemu-devel] [PATCH, RFC] trace: implement guest tracepoint passthrough

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Aug 30, 2011 at 7:43 PM, Blue Swirl <blauwirbel@gmail.com> wrote:
> On Mon, Aug 29, 2011 at 12:17 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
>> On Fri, Aug 26, 2011 at 8:06 PM, Blue Swirl <blauwirbel@gmail.com> wrote:
>>> Let guests inject tracepoint data via fw_cfg device.
>>>
>>> Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
>>> ---
>>> The patch is used like this:
>>> ../configure --with-guest-trace-file=/src/openbios-devel/trace-events
>>> make
>>> sparc64-softmmu/qemu-system-sparc64 -trace file=foo
>>> # ugly hack to combine the file, but my laziness^Wpython-fu is too
>>> weak to add handling of "--guest-trace-file
>>> /src/openbios-devel/trace-events" to simpletrace.py
>>> cat ../trace-events /src/openbios-devel/trace-events >/tmp/trace-events
>>> # examine trace file with OpenBIOS trace data with simpletrace.py
>>> ../scripts/simpletrace.py /tmp/trace-events foo
>>> ob_ide_read_blocks 0.000 dest=0xfff0bed0 blk=0x0 n=0x1
>>> ob_ide_read_blocks 6491.806 dest=0xfff0bed0 blk=0x0 n=0x1
>>>
>>> An example of a generated guest-trace.c file:
>>> /* This file is autogenerated by tracetool, do not edit. */
>>> #include "trace.h"
>>> #include "guest-trace.h"
>>>
>>> void guest_trace(uint64_t event_id, uint64_t arg1, uint64_t arg2,
>>>                 uint64_t arg3, uint64_t arg4, uint64_t arg5, uint64_t arg6)
>>>
>>> {
>>>    switch (event_id) {
>>>
>>>    case 0:
>>>        trace_esp_do_command(arg1, arg2, arg3);
>>>        break;
>>>
>>>    case 1:
>>>        trace_ob_ide_pio_insw(arg1);
>>>        break;
>>>
>>>    case 2:
>>>        trace_ob_ide_read_blocks(arg1, arg2, arg3);
>>>        break;
>>>
>>>    default:
>>>        break;
>>>   }
>>> }
>>>
>>> ---
>>>  Makefile.objs     |   15 +++++++++++-
>>>  configure         |    9 +++++++-
>>>  guest-trace.h     |    3 ++
>>>  hw/fw_cfg.c       |   31 +++++++++++++++++++++++++++
>>>  hw/fw_cfg.h       |   10 ++++++--
>>>  scripts/tracetool |   60 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>>>  6 files changed, 120 insertions(+), 8 deletions(-)
>>>  create mode 100644 guest-trace.h
>>
>> The ability to trace from the guest can be handy, so I think we should
>> have this feature.  Please add documentation on how to hook it up
>> (e.g. how people would use this for other firmware/guest code and/or
>> other architectures).
>
> OK. The format should be the same as raw simpletrace data, but words
> always in little endian like used elsewhere with fw_cfg. BTW,
> currently simpletrace file format depends on host endianness, is that
> intentional?

Yes, it is intentional.  The rationale is that we want to stream trace
events to disk as quickly as possible without any overhead.  Since
there is a magic number in the first trace record, the post-processing
tool can in theory determine the endianness and do the byteswapping at
that time.  The current simpletrace.py script does not do this.

Stefan