Re: [Qemu-devel] [RFC PATCH v2 0/7] QEMU binary instrumentation prototype

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

Pavel Dovgalyuk <dovgaluk@ispras.ru> writes:

> Peter, what about this one?
>
> Pavel Dovgalyuk
>
>> -----Original Message-----
>> From: Pavel Dovgalyuk [mailto:dovgaluk@ispras.ru]
>> Sent: Tuesday, June 05, 2018 2:56 PM
>> To: 'Peter Maydell'; 'Pavel Dovgalyuk'
>> Cc: 'QEMU Developers'; maria.klimushenkova@ispras.ru; 'Paolo Bonzini'; 'Lluís Vilanova'
>> Subject: RE: [RFC PATCH v2 0/7] QEMU binary instrumentation prototype
>>
>> > From: Peter Maydell [mailto:peter.maydell@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:
>>
>> 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.
>>
>>  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.
>>
>>  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.
>>
>> The similar pair of functions will also be added for the memory
>> operations.
>>
>> Plugin->QEMU API
>> ================
>>
>> QEMU core exports some functions to let the plugins introspect the guest
>> or perform some interaction with other QEMU services (e.g., logging).
>> API doesn't contain any data structures, because their memory layout depend
>> on the compilation settings.
>>
>> QEMU exports the following functions that may be called from the plugins:
>>
>>  1. void qemulib_log(const char *fmt, ...);
>>     Wrapper for qemu_log.
>>
>>  2. int qemulib_read_memory(void *cpu, uint64_t addr, uint8_t *buf, int len);
>>     Reads guest memory into the buffer. Wrapper for cpu_memory_rw_debug.
>>
>>  3. int qemulib_read_register(void *cpu, uint8_t *mem_buf, int reg);
>>     Uses target gdb interface for accessing the guest registers.
>>     'reg' is the id of the desired register as it is coded by gdb.
>>
>> There also should be a function for flushing the translated blocks to
>> ensure that the instrumentation will occur in the case of changing
>> the internal plugin state.

This would certainly be the case if we could generate TCG code in the plugins.

--
Alex Bennée