linux-kernel - Re: [RFC PATCH v2 00/17] objtool: Function validation tracing

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Message-ID: <4c4afe3e-ac24-4891-9acd-1d493ea88eca@oracle.com>
Date: Wed, 24 Sep 2025 11:50:25 +0200
From: Alexandre Chartre <alexandre.chartre@...cle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: alexandre.chartre@...cle.com, linux-kernel@...r.kernel.org,
        mingo@...nel.org, jpoimboe@...nel.org
Subject: Re: [RFC PATCH v2 00/17] objtool: Function validation tracing


On 9/24/25 11:17, Peter Zijlstra wrote:
> On Wed, Sep 24, 2025 at 09:36:49AM +0200, Peter Zijlstra wrote:
> 
>>>                        | <alternative.65c4e> alt 1/4 end
>>>   65c4e: os_xsave+0x2e | <alternative.65c4e> alt 2/4 begin
>>>   1c3d: .altinstr_replacement+0x1c3d | | xsaves64 0x40(%rbp)
>>>   65c53: os_xsave+0x33 | | xor %ebx,%ebx
>>>   65c55: os_xsave+0x35 | | test %ebx,%ebx - already visited
>>>                        | <alternative.65c4e> alt 2/4 end
>>>   65c4e: os_xsave+0x2e | <alternative.65c4e> alt 3/4 begin
>>>   1c38: .altinstr_replacement+0x1c38 | | xsavec64 0x40(%rbp)
>>>   65c53: os_xsave+0x33 | | xor %ebx,%ebx - already visited
>>>                        | <alternative.65c4e> alt 3/4 end
>>>   65c4e: os_xsave+0x2e | <alternative.65c4e> alt 4/4 begin
>>>   1c33: .altinstr_replacement+0x1c33 | | xsaveopt64 0x40(%rbp)
>>>   65c53: os_xsave+0x33 | | xor %ebx,%ebx - already visited
>>>                        | <alternative.65c4e> alt 4/4 end
>>>   65c4e: os_xsave+0x2e | <alternative.65c4e> alt default
>>>   65c4e: os_xsave+0x2e | xsave64 0x40(%rbp)
>>>   65c53: os_xsave+0x33 | xor %ebx,%ebx - already visited
>>
>> I find it *very* hard to read these alternatives. If at all possible, I
>> think something like:
>>
>>     65c4e: os_xsave+0x2e | xsave64	| xsaveopt64	| xsavec64	| xsaves64
>>     65c53: os_xsave+0x33 | xor %ebx,%ebx
>>
>> Would be *much* easier to follow
> 
> Another option is to write it source-like:
> 
>    65c4e: os_xsave+0x2e | ALTERNATIVE("xsave64",
>                                       "xsaveopt64", X86_FEATURE_XSAVEOPT,
> 				     "xsavec64", X86_FEATURE_XSAVEC,
> 				     "xsaves64", X86_FEATURE_XSAVES);
>    65c53: os_xsave+0x33 | xor %ebx,%ebx
> 
> 
> We have the 'feature' bit, we'd just have to 'borrow' the feature
> strings from the kernel I suppose.

Yes, that would be very useful. But I will probably look at it for a next
set of patches.

alex.