linux-kernel - Re: [PATCH v3 20/30] perf clang jit: add PerfModule::doJIT to JIT perfhook functions

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Date:   Sat, 26 Nov 2016 09:29:36 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Wang Nan <wangnan0@...wei.com>
Cc:     acme@...hat.com, ast@...com, lizefan@...wei.com,
        hekuang@...wei.com, linux-kernel@...r.kernel.org, pi3orama@....com,
        joe@....org, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v3 20/30] perf clang jit: add PerfModule::doJIT to JIT
 perfhook functions

On Sat, Nov 26, 2016 at 07:03:44AM +0000, Wang Nan wrote:
> PerfModule::doJIT JIT compile perfhook functions and saves result into
> a map. Add a test case for it.
> 
> At this stage perfhook functions can do no useful things because they
> can't invoke external functions and can't return value. Following
> commits are going to make improvment.
> 
> Don't hook functions right after jitted because bpf_object is unavailable
> during jitting but it should be the context of jitted functions.
> 
> Signed-off-by: Wang Nan <wangnan0@...wei.com>
...
> +	for (Function *F : JITFunctions) {
> +		JITSymbol sym = CompileLayer.findSymbol(F->getName().str(), true);
> +
> +		/*
> +		 * Type of F->getSection() is moving from
> +		 * const char * to StringRef.
> +		 * Convert it to std::string so we don't need
> +		 * consider this API change.
> +		 */
> +		std::string sec(F->getSection());
> +		std::string hook(&sec.c_str()[sizeof("perfhook:") - 1]);
> +		perf_hook_func_t func = (perf_hook_func_t)(intptr_t)sym.getAddress();
> +
> +		if (JITResult[hook])
> +			llvm::errs() << "Warning: multiple functions on hook "
> +				     << hook << ", only one is used\n";
> +		JITResult[hook] = func;
> +	}
> +	return 0;
> +}
> +
>  class ClangOptions {
>  	llvm::SmallString<PATH_MAX> FileName;
>  	llvm::SmallString<64> KVerDef;
> @@ -292,6 +359,10 @@ void perf_clang__init(void)
>  	LLVMInitializeBPFTarget();
>  	LLVMInitializeBPFTargetMC();
>  	LLVMInitializeBPFAsmPrinter();
> +
> +	llvm::InitializeNativeTarget();
> +	llvm::InitializeNativeTargetAsmPrinter();
> +	llvm::InitializeNativeTargetAsmParser();

Looks great. I bet a lot of people reading perf code
won't be able to understand what you're doing here.
Could you please add a design doc on how perf<->clang/llvm interaction
is done.

Acked-by: Alexei Starovoitov <ast@...nel.org>