Hi Alex, On 08/10/2018 12:28, Alex Bennée wrote: > > Emilio G. Cota <c...@braap.org> writes: > >> On Fri, Oct 05, 2018 at 16:48:49 +0100, Alex Bennée wrote: >> (snip) >>> ==Known Limitations== >>> >>> Currently there is only one hook allowed per trace event. We could >>> make this more flexible or simply just error out if two plugins try >>> and hook to the same point. What are the expectations of running >>> multiple plugins hooking into the same point in QEMU? >> >> It's very common. All popular instrumentation tools (e.g. PANDA, >> DynamoRIO, Pin) support multiple plugins. > > Fair enough. > >> >>> ==TCG Hooks== >>> >>> Thanks to Lluís' work the trace API already splits up TCG events into >>> translation time and exec time events and provides the machinery for >>> hooking a trace helper into the translation stream. Currently that >>> helper is unconditionally added although perhaps we could expand the >>> call convention a little for TCG events to allow the translation time >>> event to filter out planting the execution time helper? >> >> A TCG helper is suboptimal for these kind of events, e.g. instruction/TB/ >> mem callbacks, because (1) these events happen *very* often, and >> (2) a helper then has to iterate over a list of callbacks (assuming >> we support multiple plugins). That is, one TCG helper call, >> plus cache misses for the callback pointers, plus function calls >> to call the callbacks. That adds up to 2x average slowdown >> for SPEC06int, instead of 1.5x slowdown when embedding the >> callbacks directly into the generated code. Yes, you have to >> flush the code when unsubscribing from the event, but that cost >> is amortized by the savings you get when the callbacks occur, >> which are way more frequent. > > What would you want instead of a TCG helper? But certainly being able be > selective about which instances of each trace point are instrumented > will be valuable. > >> Besides performance, to provide a pleasant plugin experience we need >> something better than the current tracing callbacks. > > What I hope to avoid in re-using trace points is having a whole bunch of > a additional hook points just for plugins. However nothing stops us > adding more tracepoints at more useful places for instrumentation. We > could also do it on a whitelist basis similar to the way the tcg events > are marked. > >> >>> ===Instruction Tracing=== >>> >>> Pavel's series had a specific hook for instrumenting individual >>> instructions. I have not yet added it to this series but I think it be >>> done in a slightly cleaner way now we have the ability to insert TCG >>> ops into the instruction stream. >> >> I thought Peter explicitly disallowed TCG generation from plugins. >> Also, IIRC others also mentioned that exposing QEMU internals >> (e.g. "struct TranslationBlock", or "struct CPUState") to plugins >> was not on the table. > > We definitely want to avoid plugin controlled code generation but the > tcg tracepoint mechanism is transparent to the plugin itself. I think > the pointers should really be treated as anonymous handles rather than > windows into QEMU's internals. Arguably some of the tracepoints should > be exporting more useful numbers (I used cpu->cpu_index in the TLB trace > points) but I don't know if we can change existing trace point > definitions to clean that up. > > Again if we whitelist tracepoints for plugins we can be more careful > about the data exported. > >> >>> If we add a tracepoint for post >>> instruction generation which passes a buffer with the instruction >>> translated and method to insert a helper before or after the >>> instruction. This would avoid exposing the cpu_ldst macros to the >>> plugins. >> >> Again, for performance you'd avoid the tracepoint (i.e. calling >> a helper to call another function) and embed directly the >> callback from TCG. Same thing applies to TB's. > > OK I see what you mean. I think that is doable although it might take a > bit more tcg plumbing. > >> >>> So what do people think? Could this be a viable way to extend QEMU >>> with plugins? >> >> For frequent events such as the ones mentioned above, I am >> not sure plugins can be efficiently implemented under >> tracing. > > I assume some form of helper-per-instrumented-event/insn is still going > to be needed though? We are not considering some sort of EBF craziness? > >> For others (e.g. cpu_init events), sure, they could. >> But still, differently from tracers, plugins can come and go >> anytime, so I am not convinced that merging the two features >> is a good idea. > > I don't think we have to mirror tracepoints and plugin points but I'm in > favour of sharing the general mechanism and tooling rather than having a > whole separate set of hooks. We certainly don't want anything like: > > trace_exec_tb(tb, pc); > plugin_exec_tb(tb, pc); > > scattered throughout the code where the two do align.
What about turning the tracepoints into the default instrumentation plugin? (the first of Emilio's list of plugins). >> >> Thanks, >> >> Emilio > > > -- > Alex Bennée >