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    SubjectRe: [PATCH v5] perf machine: arm/arm64: Improve completeness for kernel address space
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    On 15/08/19 2:32 PM, Leo Yan wrote:
    > Hi Adrian,
    >
    > On Thu, Aug 15, 2019 at 11:54:54AM +0300, Adrian Hunter wrote:
    >
    > [...]
    >
    >>> diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
    >>> index e4988f49ea79..d7ff839d8b20 100644
    >>> --- a/tools/perf/Makefile.config
    >>> +++ b/tools/perf/Makefile.config
    >>> @@ -48,9 +48,20 @@ ifeq ($(SRCARCH),x86)
    >>> NO_PERF_REGS := 0
    >>> endif
    >>>
    >>> +ARM_PRE_START_SIZE := 0
    >>> +
    >>> ifeq ($(SRCARCH),arm)
    >>> NO_PERF_REGS := 0
    >>> LIBUNWIND_LIBS = -lunwind -lunwind-arm
    >>> + ifneq ($(wildcard $(srctree)/arch/$(SRCARCH)/kernel/vmlinux.lds),)
    >>> + # Extract info from lds:
    >>> + # . = ((0xC0000000)) + 0x00208000;
    >>> + # ARM_PRE_START_SIZE := 0x00208000
    >>> + ARM_PRE_START_SIZE := $(shell egrep ' \. \= \({2}0x[0-9a-fA-F]+\){2}' \
    >>> + $(srctree)/arch/$(SRCARCH)/kernel/vmlinux.lds | \
    >>> + sed -e 's/[(|)|.|=|+|<|;|-]//g' -e 's/ \+/ /g' -e 's/^[ \t]*//' | \
    >>> + awk -F' ' '{printf "0x%x", $$2}' 2>/dev/null)
    >>> + endif
    >>> endif
    >>>
    >>> ifeq ($(SRCARCH),arm64)
    >>> @@ -58,8 +69,19 @@ ifeq ($(SRCARCH),arm64)
    >>> NO_SYSCALL_TABLE := 0
    >>> CFLAGS += -I$(OUTPUT)arch/arm64/include/generated
    >>> LIBUNWIND_LIBS = -lunwind -lunwind-aarch64
    >>> + ifneq ($(wildcard $(srctree)/arch/$(SRCARCH)/kernel/vmlinux.lds),)
    >>> + # Extract info from lds:
    >>> + # . = ((((((((0xffffffffffffffff)) - (((1)) << (48)) + 1) + (0)) + (0x08000000))) + (0x08000000))) + 0x00080000;
    >>> + # ARM_PRE_START_SIZE := (0x08000000 + 0x08000000 + 0x00080000) = 0x10080000
    >>> + ARM_PRE_START_SIZE := $(shell egrep ' \. \= \({8}0x[0-9a-fA-F]+\){2}' \
    >>> + $(srctree)/arch/$(SRCARCH)/kernel/vmlinux.lds | \
    >>> + sed -e 's/[(|)|.|=|+|<|;|-]//g' -e 's/ \+/ /g' -e 's/^[ \t]*//' | \
    >>> + awk -F' ' '{printf "0x%x", $$6+$$7+$$8}' 2>/dev/null)
    >>> + endif
    >>
    >> So, that is not going to work if you take a perf.data file to a non-arm machine?
    >
    > Yeah, this patch will only allow perf to work correctly when perf
    > run natively on arm/arm64, so it can resolve partial of the issue.
    >
    >> 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?

    > so I finally gave up to use
    > machine__create_extra_kernel_maps() for tweaking kernel start address
    > and went back to use this patch's approach by parsing lds files.
    >
    > So for next step, I want to get some guidances:
    >
    > - One method is to add a new weak function, e.g.
    > arch__fix_kernel_text_start(), then every arch can implement its own
    > function to fixup the kernel start address;
    >
    > For arm/arm64, can use kallsyms to find the symbols with least
    > address and fixup for kernel start address.
    >
    > - Another method is to directly parse kallsyms in the function
    > machine__get_kernel_start(), thus the change can be used for all
    > archs;
    >
    > Seems to me the second method is to address this issue as a common
    > issue crossing all archs. But not sure if this is the requirement for
    > all archs or just this is only required for arm/arm64. Please let me
    > know what's your preference or other thoughts. Thanks a lot!
    >
    > Leo.
    >
    >>> endif
    >>>
    >>> +CFLAGS += -DARM_PRE_START_SIZE=$(ARM_PRE_START_SIZE)
    >>> +
    >>> ifeq ($(SRCARCH),csky)
    >>> NO_PERF_REGS := 0
    >>> endif
    >>> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
    >>> index f6ee7fbad3e4..e993f891bb82 100644
    >>> --- a/tools/perf/util/machine.c
    >>> +++ b/tools/perf/util/machine.c
    >>> @@ -2687,13 +2687,26 @@ int machine__get_kernel_start(struct machine *machine)
    >>> machine->kernel_start = 1ULL << 63;
    >>> if (map) {
    >>> err = map__load(map);
    >>> + if (err)
    >>> + return err;
    >>> +
    >>> /*
    >>> * On x86_64, PTI entry trampolines are less than the
    >>> * start of kernel text, but still above 2^63. So leave
    >>> * kernel_start = 1ULL << 63 for x86_64.
    >>> */
    >>> - if (!err && !machine__is(machine, "x86_64"))
    >>> + if (!machine__is(machine, "x86_64"))
    >>> machine->kernel_start = map->start;
    >>> +
    >>> + /*
    >>> + * On arm/arm64, the kernel uses some memory regions which are
    >>> + * prior to '_stext' symbol; to reflect the complete kernel
    >>> + * address space, compensate these pre-defined regions for
    >>> + * kernel start address.
    >>> + */
    >>> + if (!strcmp(perf_env__arch(machine->env), "arm") ||
    >>> + !strcmp(perf_env__arch(machine->env), "arm64"))
    >>> + machine->kernel_start -= ARM_PRE_START_SIZE;
    >>> }
    >>> return err;
    >>> }
    >>>
    >>
    >

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