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Re: [Qemu-devel] RFC: Design Doc for a new trace format (to support vari

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] RFC: Design Doc for a new trace format (to support variable number/size of args per event) simpletrace-v2
Date: Tue, 29 Nov 2011 11:47:41 +0000

On Tue, Nov 29, 2011 at 11:34 AM, Stefan Hajnoczi <address@hidden> wrote:
> On Tue, Nov 29, 2011 at 8:29 AM, Harsh Bora <address@hidden> wrote:
>> Currently, Qemu provides an in-built "simple" trace backend which is simple
>> and easy to use (no additional/external dependencies) and allows developers
>> to trace events in Qemu code, however, it suffers from limitations like
>> unability to trace more than 6 elements per trace event, lack of string
>> support, etc. There are various places in Qemu code, where one would want to
>> trace events having multiple arguments including strings. This results into
>> motivation for defining an advanced, yet simple trace format which will
>> address these limitations. For the sake of convinence, let us call this new
>> trace format as simpletrace v2 (any other better name?).
>> HLD for Simpletrace v2:
>> ======================
>> This new trace format defines 3 types of structures for trace data
>> organization:
>> 1) Trace Log Header (per log file, provides meta-data about the entire trace
>> log, like trace format version, endianness, etc.)
>> 2) Trace Event Header (per trace event, provides meta-data per trace event)
>> 3) Trace Data (per argument/data in a trace event, provides size of data
>> followed by data itself)
>>  The Trace Log header can be defined like this:
>> typedef struct {
>>        uint64_t endian_magic;  /* =0xAA0011FF, a magic number helps
>> identifying validity and endian-ness of trace log to its readers */
>>        uint64_t pid;           /* pid of qemu process can be traced here */
>>        uint64_t version;       /* Keeping version info as 3rd 64-bit element
>> as expected by current simpletrace format */
>>        unit64_t timestamp;     /* timestamp info */
>> } TraceLogHeader;
>> Suggestions are invited to make this header more informative as required.
>> Further, this TraceLogHeader will be followed by 0 or more Trace Event
>> Headers (further followed by trace data) as defined below:
>> typedef struct {
>>        uint64_t magic;         /* =0xA1B2C3D4, ensures a valid trace record,
>> otherwise corrupted */
>>        unit64_t id;            /* unique identifer per trace event */
>>        uint64_t timestamp;     /* timestamp info */
>>        uint64_t num_args;      /* number of arguments (followed by this
>> TraceEventHeader) traced in this trace event */
> What exactly is num_args?
>> } TraceEventHeader;
>> Trace Data is expected to be stored in following format followed by
>> TraceEventHeader:
>> typedef struct {
>>        uint64_t size;  /* size of data in bytes to be read following this
>> header */
>>        uint8_t data[0] /* variable sized data */
>> } TraceData;
> This format does not support variable-length strings in a
> self-describing way.

I used the word "self-describing".  To be clear I think we should not
put any trace event metadata into the trace file.  Instead we continue
to rely on having the trace-events file together with the trace file.

This keeps the trace format simple.  If we do want a self-describing
trace format then I think we should use the Common Trace Format that
LTTng is working on instead of rolling our own.

So forget I said "self-describing" :).  I think the only changes from
the v1 format we need are:

1. New magic number to mark v2 format.

2. Trace records are no longer fixed-length, they include a size field:

typedef struct {
    uint32_t length; /* in bytes */
    uint32_t reserved; /* unused */
    uint64_t event;
    uint64_t timestamp_ns;
    uint8_t arguments[];
} TraceRecord;

3. Strings are serialized like this:

uint16_t length;
char chars[length];

These changes enable:

1. Variable number of trace event arguments.

2. Variable-length strings.


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