Re: [PATCH] ARM64: perf: support dwarf unwinding in compat mode

From: Jean Pihet
Date: Thu Jan 16 2014 - 08:47:16 EST


On 16 January 2014 13:57, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Thu, Jan 16, 2014 at 12:26:53PM +0000, Jean Pihet wrote:
>> On 16 January 2014 12:56, Will Deacon <will.deacon@xxxxxxx> wrote:
>> > In your previous series, compat backtracing is actually split out into a
>> > separate function (compat_user_backtrace), so it would be more consistent to
>> > have a compat_user_stack_pointer macro, rather than add this check here.
The compat_user_backtrace function is used to unwind using the frame
pointer, it is not used to unwind using the dwarf info (which uses the
user stack pointer).

>> Do you mean this change instead?
> I don't think so...
>> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
>> index 569b2187..9b88d2e 100644
>> --- a/kernel/events/internal.h
>> +++ b/kernel/events/internal.h
>> @@ -185,7 +185,8 @@ static inline bool arch_perf_have_user_stack_dump(void)
>> return true;
>> }
>> -#define perf_user_stack_pointer(regs) user_stack_pointer(regs)
>> +#define perf_user_stack_pointer(regs) \
>> + (!compat_user_mode(regs)) ? ((regs)->sp) : ((regs)->compat_sp)
> This doesn't belong in core code; compat_user_mode and the fields of regs
> are arm64-specific.

> So I suppose you need to rework your original patch to
> call compat_user_stack_pointer (which we already define in compat.h for
> arm64) if compat_user_mode(regs)).
The perf core code calls perf_user_stack_pointer(regs) to retrieve the
stack pointer, with perf_user_stack_pointer(regs) defined as
The problem is that perf is not aware of the compat mode, so every
arch has to implement user_stack_pointer(regs) correctly.

For this reason I think the first patch proposal is the right one
unless the perf core code is redesigned to handle different ABIs. Do
you see a better implementation?

> The problem there is the inconsistency with respect to the regs argument:
> user_stack_pointer(regs) // Returns user stack pointer for regs
> current_user_stack_pointer() // Returns current user stack pointer
> compat_user_stack_pointer() // Doesn't take a regs argument!
> On top of that, x86 treats those last two functions differently when current
> is a compat task.
> So the simplest thing would be to make compat_user_stack_pointer expand to
> user_stack_pointer(current_pt_regs()) on arm64 and merge that in with your
> original patch fixing user_stack_pointer.
> Will

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