trace: [tcg] Add documentation

You can check both if the event has been disabled and is dynamically enabled at

the same time using the 'trace_event_get_state' routine (see header

"trace/control.h" for more information).

=== "tcg" ===

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

"<eventname>_exec" to trace the event at execution time.

Instead of using these two events, you should instead use the function

"trace_<eventname>_tcg" during translation (TCG code generation). This function

will automatically call "trace_<eventname>_trans", and will generate the

necessary TCG code to call "trace_<eventname>_exec" during guest code execution.

Events with the "tcg" property can be declared in the "trace-events" file with a

mix of native and TCG types, and "trace_<eventname>_tcg" will gracefully forward

them to the "<eventname>_trans" and "<eventname>_exec" events. Since TCG values

are not known at translation time, these are ignored by the "<eventname>_trans"

event. Because of this, the entry in the "trace-events" file needs two printing

formats (separated by a comma):

    tcg foo(uint8_t a1, TCGv_i32 a2) "a1=%d", "a1=%d a2=%d"

For example:

    #include "trace-tcg.h"

    void some_disassembly_func (...)

    {

        uint8_t a1 = ...;

        TCGv_i32 a2 = ...;

        trace_foo_tcg(a1, a2);

    }

This will immediately call:

    void trace_foo_trans(uint8_t a1);

and will generate the TCG code to call:

    void trace_foo(uint8_t a1, uint32_t a2);