Re: [PATCH v1 5/7] docs: mark intention to deprecate TCG tracing functionality

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

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Wed, May 05, 2021 at 10:22:57AM +0100, Alex Bennée wrote:
>> Currently attempts to add a new TCG trace events results in failures
>> to build. Previous discussions have suggested maybe it's time to mark
>> the feature as deprecated and push people towards using plugins.
>> 
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> Cc: Luis Vilanova <vilanova@imperial.ac.uk>
>> Cc: Stefan Hajnoczi <stefanha@redhat.com>
>> ---
>>  docs/devel/tcg-plugins.rst |  2 ++
>>  docs/devel/tracing.rst     |  7 +++++++
>>  docs/system/deprecated.rst | 13 +++++++++++++
>>  3 files changed, 22 insertions(+)
>> 
>> diff --git a/docs/devel/tcg-plugins.rst b/docs/devel/tcg-plugins.rst
>> index 18c6581d85..edf04e3091 100644
>> --- a/docs/devel/tcg-plugins.rst
>> +++ b/docs/devel/tcg-plugins.rst
>> @@ -3,6 +3,8 @@
>>     Copyright (c) 2019, Linaro Limited
>>     Written by Emilio Cota and Alex Bennée
>>  
>> +.. _tcgplugin-ref:
>> +
>>  ================
>>  QEMU TCG Plugins
>>  ================
>> diff --git a/docs/devel/tracing.rst b/docs/devel/tracing.rst
>> index ba83954899..6b0f46cd54 100644
>> --- a/docs/devel/tracing.rst
>> +++ b/docs/devel/tracing.rst
>> @@ -414,6 +414,13 @@ disabled, this check will have no performance impact.
>>  "tcg"
>>  -----
>>  
>> +.. warning::
>> +   The ability to add new TCG trace points relies on a having a good
>> +   understanding of the TCG internals. In the meantime TCG plugins
>> +   have been introduced which solve many of the same problems with
>> +   more of a focus on analysing guest code. See :ref:`tcgplugin-ref`
>> +   for more details.
>> +
>>  Guest code generated by TCG can be traced by defining an event with the "tcg"
>>  event property. Internally, this property generates two events:
>>  "<eventname>_trans" to trace the event at translation time, and
>> diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
>> index 80cae86252..0c9d3c1e1e 100644
>> --- a/docs/system/deprecated.rst
>> +++ b/docs/system/deprecated.rst
>> @@ -312,6 +312,19 @@ The ``I7200`` guest CPU relies on the nanoMIPS ISA, which is deprecated
>>  (the ISA has never been upstreamed to a compiler toolchain). Therefore
>>  this CPU is also deprecated.
>>  
>> +TCG introspection features
>> +--------------------------
>> +
>> +TCG trace-events (since 6.1)
>> +''''''''''''''''''''''''''''
>> +
>> +The ability to add new TCG trace points has bit rotted and as the
>
> When you say this "has bit rotted", just how bad is the situation ?
>
> Is the TCG tracing still usable at all, or is is fully broken
> already ?

Well patches 6/7 got it working for generic TCG things. I haven't been
able to get the architecture one working but I suspect that is some sort
of interaction between the per-arch trace header generation that I
haven't quite figured out yet.

It's currently broken without the included patches because it's not
really being exercised by anything.

>> +feature can be replicated with TCG plugins it will be deprecated. If
>> +any user is currently using this feature and needs help with
>> +converting to using TCG plugins they should contact the qemu-devel
>> +mailing list.
>> +
>
> Regards,
> Daniel


-- 
Alex Bennée