netdev - Re: [PATCH v5] perf machine: arm/arm64: Improve completeness for kernel address space

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Date:   Fri, 16 Aug 2019 09:45:41 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, clang-built-linux@...glegroups.com,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Suzuki Poulouse <suzuki.poulose@....com>,
        coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5] perf machine: arm/arm64: Improve completeness for
 kernel address space

Hi Adrian,

On Thu, Aug 15, 2019 at 02:45:57PM +0300, Adrian Hunter wrote:

[...]

> >> How come you cannot use kallsyms to get the information?
> > 
> > Thanks for pointing out this.  Sorry I skipped your comment "I don't
> > know how you intend to calculate ARM_PRE_START_SIZE" when you reviewed
> > the patch v3, I should use that chance to elaborate the detailed idea
> > and so can get more feedback/guidance before procceed.
> > 
> > Actually, I have considered to use kallsyms when worked on the previous
> > patch set.
> > 
> > As mentioned in patch set v4's cover letter, I tried to implement
> > machine__create_extra_kernel_maps() for arm/arm64, the purpose is to
> > parse kallsyms so can find more kernel maps and thus also can fixup
> > the kernel start address.  But I found the 'perf script' tool directly
> > calls machine__get_kernel_start() instead of running into the flow for
> > machine__create_extra_kernel_maps();
> 
> Doesn't it just need to loop through each kernel map to find the lowest
> start address?

Based on your suggestion, I worked out below change and verified it
can work well on arm64 for fixing up start address; please let me know
if the change works for you?

diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index f6ee7fbad3e4..51d78313dca1 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -2671,9 +2671,26 @@ int machine__nr_cpus_avail(struct machine *machine)
 	return machine ? perf_env__nr_cpus_avail(machine->env) : 0;
 }
 
+static int machine__fixup_kernel_start(void *arg,
+				       const char *name __maybe_unused,
+				       char type,
+				       u64 start)
+{
+	struct machine *machine = arg;
+
+	type = toupper(type);
+
+	/* Fixup for text, weak, data and bss sections. */
+	if (type == 'T' || type == 'W' || type == 'D' || type == 'B')
+		machine->kernel_start = min(machine->kernel_start, start);
+
+	return 0;
+}
+
 int machine__get_kernel_start(struct machine *machine)
 {
 	struct map *map = machine__kernel_map(machine);
+	char filename[PATH_MAX];
 	int err = 0;
 
 	/*
@@ -2687,6 +2704,7 @@ int machine__get_kernel_start(struct machine *machine)
 	machine->kernel_start = 1ULL << 63;
 	if (map) {
 		err = map__load(map);
 		/*
 		 * On x86_64, PTI entry trampolines are less than the
 		 * start of kernel text, but still above 2^63. So leave
@@ -2695,6 +2713,16 @@ int machine__get_kernel_start(struct machine *machine)
 		if (!err && !machine__is(machine, "x86_64"))
 			machine->kernel_start = map->start;
 	}
+
+	machine__get_kallsyms_filename(machine, filename, PATH_MAX);
+
+	if (symbol__restricted_filename(filename, "/proc/kallsyms"))
+		goto out;
+
+	if (kallsyms__parse(filename, machine, machine__fixup_kernel_start))
+		pr_warning("Fail to fixup kernel start address. skipping...\n");
+
+out:
 	return err;
 }

Thanks,
Leo Yan