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

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Message-ID: <20250924091751.GX4068168@noisy.programming.kicks-ass.net>
Date: Wed, 24 Sep 2025 11:17:51 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Alexandre Chartre <alexandre.chartre@...cle.com>
Cc: linux-kernel@...r.kernel.org, mingo@...nel.org, jpoimboe@...nel.org
Subject: Re: [RFC PATCH v2 00/17] objtool: Function validation tracing

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.