Re: [PATCH v8 06/12] unwind_user/sframe: Wire up unwind_user to sframe

[Jens Remus <jremus@linux.ibm.com>]

On 09.07.2025 15:51, Mathieu Desnoyers wrote:
> On 2025-07-09 09:46, Mathieu Desnoyers wrote:
>> I concur with Jens. I think we should keep track of both:
>>
>> 1) available unwind methods,
>>
>> 2) unwind method used for the current frame.
>>
>> E.g.:
>>
>> /*
>>   * unwind types, listed in priority order: lower numbers are
>>   * attempted first if available.
>>   */
>> enum unwind_user_type_bits {
>>          UNWIND_USER_TYPE_SFRAME_BIT = 0,
>>          UNWIND_USER_TYPE_FP_BIT = 1,
>>          UNWIND_USER_TYPE_COMPAT_FP_BIT = 2,
>>
>>      _NR_UNWIND_USER_TYPE_BITS,
>> };
>>
>> enum unwind_user_type {
>>          UNWIND_USER_TYPE_NONE = 0,
>>          UNWIND_USER_TYPE_SFRAME = (1U << UNWIND_USER_TYPE_SFRAME_BIT),
>>          UNWIND_USER_TYPE_FP = (1U << UNWIND_USER_TYPE_FP_BIT),
>>          UNWIND_USER_TYPE_COMPAT_FP = (1U <<  UNWIND_USER_TYPE_COMPAT_FP_BIT),
>> };
>>
>> And have the following fields in struct unwind_user_state:
>>
>> /* Unwind time used for the most recent unwind traversal iteration. */
>> enum unwind_user_type current_type;
>>
>> /* Unwind types available in the current context. Bitmask of enum unwind_user_type. */
>> unsigned int available_types;
>>
>> So as we end up adding stuff like registered JIT unwind info, we will
>> want to expand the "available types". And it makes sense to both keep
>> track of all available types (as a way to quickly know which mechanisms
>> we need to query for the current task) *and* to let the caller know
>> which unwind type was used for the current frame.
>>
>> And AFAIU we'd be inserting a "jit unwind info" type between SFRAME and FP in
>> the future, because the jit unwind info would be more reliable than FP. This
>> would require that we bump the number for FP and COMPAT_FP, but that would
>> be OK because this is not ABI.
>>
>> Thoughts ?
> 
> One use-case for giving the "current_type" to iteration callers is to
> let end users know whether they should trust the frame info. If it
> comes from sframe, then it should be pretty solid. However, if it comes
> from frame pointers used as a fallback on a system that omits frame
> pointers, the user should consider the resulting data with a high level
> of skepticism.

The current_type may be different for every unwind step (frame).  So
struct unwind_stacktrace would probably need the following added:

	/* Unwind types used for taking the stack trace.  */ 
	unsigned int used_types;

So that the user of unwind_user() could decide whether to trust the
stack trace.  But as Steve suggested in his reply, all of this could
be added later, once there is a need.

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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