Re: [PATCH v2 2/2] docs/devel: tvg-plugins: add execlog plugin description

["Alex Bennée" <alex.bennee@linaro.org>]

Alexandre IOOSS <erdnaxe@crans.org> writes:

> [[PGP Signed Part:Undecided]]
> On 6/22/21 12:37 PM, Alex Bennée wrote:
>> Alexandre IOOSS <erdnaxe@crans.org> writes:
>> 
>>> [[PGP Signed Part:Undecided]]
>>> On 6/22/21 10:48 AM, Alex Bennée wrote:
>>>> Alexandre Iooss<erdnaxe@crans.org>  writes:
>>>>> [...]
>>>>> +
>>>>> +The execlog tool traces executed instructions with memory access. It can be used
>>>>> +for debugging and security analysis purposes.
>>>> We should probably mention that this will generate a lot of output.
>>>> Running the admittedly memory heavy softmmu memory test:
>>>>     ./aarch64-softmmu/qemu-system-aarch64 -D test.out -d plugin \
>>>>       -plugin contrib/plugins/libexeclog.so  \
>>>>       -cpu max -serial mon:stdio -M virt \
>>>>       -display none -semihosting-config chardev=serial0 \
>>>>       -kernel ./tests/tcg/aarch64-softmmu/memory
>>>> generates a 8.6Gb text file. I suspect once this is merged you might
>>>> want to look at options to target the instrumentation at areas of
>>>> specific interest or abbreviate information.
>>>
>>> Yes! In my downstream version I am triggering the beginning and the
>>> end of trace acquisition by matching two virtual addresses of GPIO
>>> device access. This works in my case because I'm also using the same
>>> GPIO for triggering an oscilloscope, but maybe we would like to
>>> upstream something more generic.
>>>
>>> I'm still thinking about this (maybe for a later patch) but I believe
>>> it would be nice to have the following:
>>>   - If no argument is given to the plugin, log everything.
>>>   - Allow the user to specify either a memory address, an instruction
>>>     virtual address or an opcode that would start the acquisition.
>>>   - Same to stop the acquisition.
>> Sounds reasonable to me.
>> 
>>> This would look like this to start/stop acquisition using GPIO PA8 on
>>> STM32VLDISCOVERY:
>>>
>>>    ./arm-softmmu/qemu-system-arm -M stm32vldiscovery \
>>>      -kernel ./firmware.elf -d plugin \
>>>      -plugin libexeclog.so,arg=mem:1073809424,arg=mem:1073809424
>> I quite like the formats you can use for -dfilter, for example:
>>    0x1000+0x100,0x2100-0x100,0x3000..0x3100
>> it might even be worth exposing qemu_set_dfilter_ranges as a helper
>> function to plugins to avoid copy and paste.
>
> We could expose "-dfilter", but maybe it is better to reserve it to
> filter the output of the plugin rather than triggering the tracing?

I meant the parsing code for dfilter style expressions, the dfilter
itself ;-)

> I could implement a format similar to dfilter to configure triggering.
> This would enable someone to start logging on any access to a memory
> range.
>
>> So what would your above command trigger? A write to 1073809424
>> would
>> start the trace and the next write to the same address would stop it?
>> 
>
> Yes exactly. In this case the first access set the GPIO high, and the
> second access set it low.
>
> I don't believe the plugin can access the value stored in memory (i.e.
> differentiating between setting a GPIO output high or low). I don't
> find this problematic in my case, but maybe it could be for someone
> else.

Not currently but in principle it wouldn't be too hard to do. It would
just be extra data to copy into a TCG Arg. We would probably want to
make it optional though.

>
> From the discussion I see the following possible patches:
> 1. Add an argument to trigger the beginning with one address (memory
> or instruction).
> 2. Add an argument to trigger the end with one address (memory or
> instruction).
> 3. Add the support for ranges (in "dfilter" style).
> 4. (maybe) Add the support to trigger on an opcode.
> 5. Add support for "-dfilter" to filter the logging output.
>
> Thanks,
> -- Alexandre
>
> [[End of PGP Signed Part]]


-- 
Alex Bennée