Peter Maydell <peter.mayd...@linaro.org> writes:
> Alex, were you planning to look at this series ? Yes it's in my queue. I'll be looking at it as part of the presentation I'm doing for Connect. > > thanks > -- PMM > > On 29 August 2018 at 06:39, Pavel Dovgalyuk <dovga...@ispras.ru> wrote: >> Ping? >> >> >> >> Pavel Dovgalyuk >> >>> -----Original Message----- >>> From: Stefan Hajnoczi [mailto:stefa...@gmail.com] >>> Sent: Tuesday, July 10, 2018 4:07 PM >>> To: Pavel Dovgalyuk >>> Cc: 'Peter Maydell'; 'Pavel Dovgalyuk'; 'Paolo Bonzini'; >>> maria.klimushenk...@ispras.ru; 'QEMU >>> Developers'; 'Lluís Vilanova' >>> Subject: Re: [Qemu-devel] [RFC PATCH v2 0/7] QEMU binary instrumentation >>> prototype >>> >>> On Tue, Jun 05, 2018 at 02:56:29PM +0300, Pavel Dovgalyuk wrote: >>> > > From: Peter Maydell [mailto:peter.mayd...@linaro.org] >>> > > >>> > > This series doesn't seem to add anything to Documentation/ that >>> > > describes the API we make available to plugins. I'm a lot more >>> > > interested in reviewing the API that will be used by plugins >>> > > than I am in the implementation at this stage. Can you provide >>> > > a description/documentation of the API for review, please? >>> > >>> > >>> > Here is the draft: >>> >>> I like the minimal interface that you are proposing and that it is >>> completely separate from QEMU-internal APIs. This will make it easy to >>> keep this public API cleanly separated from private internal APIs. >>> >>> > Introduction >>> > ============ >>> > >>> > This document describes an API for creating the QEMU >>> > instrumentation plugins. >>> > >>> > It is based on the following prior sources: >>> > - KVM Forum 2017 talk "Instrumenting, Introspection, and Debugging with >>> > QEMU" >>> > https://www.linux-kvm.org/images/3/3d/Introspect.pdf >>> > - Discussion on Lluis Vilanova instrumentation patch series >>> > https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg03357.html >>> > >>> > The aim of the instrumentation is implementing different runtime >>> > tracers that can track the executed instructions, memory and >>> > hardware operations. >>> > >>> > Instrumenting the code >>> > ====================== >>> > >>> > Instrumentation subsystem exploits TCG helper mechanism to embed >>> > callbacks into the translation blocks. These callbacks may be inserted >>> > before the specific instructions, when the plugins require such filtering. >>> > >>> > Translator uses two functions for embedding the callbacks: >>> > - first function checks whether the current instruction should be >>> > instrumented >>> > - second function embeds the callback for executing the plugin-specific >>> > code before that instruction >>> > >>> > The similar method may be used for memory access instrumentation. >>> > >>> > QEMU->Plugin API >>> > ================ >>> > >>> > Instrumentation layer passes the requests from the translator >>> > to the dynamically loaded plugins. Every plugin may provide >>> > the following functions to perform the instrumentation: >>> > >>> > 1. bool plugin_init(const char *args); >>> > Initialization function. May return false if the plugin >>> > can't work in the current environment. >>> >>> Please document how plugin loading and argument handling works. >>> >>> Do you think unloading is necessary? For example, on a long-running >>> guest it could be useful to unload the plugin, modify and recompile it, >>> and then load it again during development. And maybe unloading is also >>> useful in cases where a plugin produces a lot of data or slows down >>> execution of a long-running guest. >>> >>> > >>> > 2. bool plugin_needs_before_insn(uint64_t pc, void *cpu); >>> > Returns true if the plugin needs to instrument the current >>> > instruction. >>> > It may use the address (pc) for making the decision or the guest >>> > CPU state (cpu), which can be passed back to QEMU core API >>> > (e.g., for reading the guest memory). >>> > This function is called at both translation and execution phases. >>> >>> What type of address is 'pc' - guest virtual or guest physical? >>> >>> Is the guest CPU state well-defined when this function is called? For >>> example, is reading CPU registers meaningful in this function since it >>> could be called at pretty much any time? >>> >>> Why is this function called during execution? I expected this to be >>> called at translation time only. If a plugin decides at runtime to >>> instrument instructions that were previously not instrumented, then it >>> could flush the relevant TB(s) - that seems a lot more efficient than >>> calling this function for every instruction that gets executed. But >>> maybe I am missing a use case for calling this at execution time...? >>> >>> > 3. void plugin_before_insn(uint64_t pc, void *cpu); >>> > If the previous function returned true for some instruction, >>> > then this function will be called. This process is repeated before >>> > every execution of the instruction, if it was instrumented. >>> >>> Plugins that instrument multiple kinds of instructions will have to >>> first look up pc and decide which kind of instruction it is. The plugin >>> could keep a list or hash table, or it could read memory to check the >>> guest code again. This will be very repetitive - many plugins will need >>> to do this. >>> >>> A slightly different take on this API is: >>> >>> /* Plugin->QEMU API */ >>> >>> /* Called by QEMU before translating an instruction >>> * @pc: guest virtual address of instruction >>> */ >>> void plugin_pre_translate(void *cpu, uint64_t pc); >>> >>> /* QEMU->Plugin API */ >>> >>> /* A callback invoked by QEMU before executing an instrumented >>> * instruction >>> * @opaque: plugin-specific data >>> */ >>> typedef void (*InstrumentCallback)(void *cpu, void *opaque); >>> >>> /* Register a callback @cb each time the instruction at @pc is about >>> * to be executed >>> * @cpu: the cpu to instrument or NULL to instrument all cpus >>> * @opaque: plugin-specific data that is passed to @cb >>> */ >>> void instrument(void *cpu, uint64_t pc, >>> InstrumentCallback cb, >>> void *opaque); >>> >>> /* Unregister a callback @cb previously registered using instrument() >>> */ >>> void uninstrument(void *cpu, uint64_t pc, >>> InstrumentCallback cb, >>> void *opaque); >>> >>> Here plugin_pre_translate() is similar to plugin_needs_before_insn(), >>> but note it has no return value. Instead of telling QEMU whether or not >>> to instrument an instruction, it must call instrument() if it wishes to >>> receive a callback immediately before a particular instruction is >>> executed. >>> >>> This is just an idea I wanted to share. You understand the use cases >>> for binary instrumentation much better than me. Feel free to disregard >>> if it doesn't fit. >> -- Alex Bennée
