Re: [Qemu-devel] RFC: Design Doc for a new trace format (to support variable number/size of args per event) simpletrace-v2

[Harsh Bora <harsh@linux.vnet.ibm.com>]

On 11/29/2011 06:26 PM, Stefan Hajnoczi wrote:
> On Tue, Nov 29, 2011 at 12:35 PM, Markus Armbruster<armbru@redhat.com>  wrote:
>> Stefan Hajnoczi<stefanha@gmail.com>  writes:
>>
>> [...]
>>> 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];
>>
>> 16 bit length?  Sure you want that?
>
> You're right, it's more future-proof to use a larger type.  Since I
> said uint32_t length in TraceRecord, uint32_t here would work well.

Makes sense to store size info only for strings as long as we rely on 
trace-events file to identify other meta-data about trace-events.

thanks,
Harsh

>
> Stefan
>