linux-kernel - Re: [RFC PATCH v2 16/16] [DO NOT MERGE] x86/kexec: enable DEBUG

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Message-ID: <Z0RBl6pzNf5irop9@gmail.com>
Date: Mon, 25 Nov 2024 10:21:27 +0100
From: Ingo Molnar <mingo@...nel.org>
To: David Woodhouse <dwmw2@...radead.org>
Cc: kexec@...ts.infradead.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Kai Huang <kai.huang@...el.com>,
	Nikolay Borisov <nik.borisov@...e.com>,
	linux-kernel@...r.kernel.org, Simon Horman <horms@...nel.org>,
	Dave Young <dyoung@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, jpoimboe@...nel.org
Subject: Re: [RFC PATCH v2 16/16] [DO NOT MERGE] x86/kexec: enable DEBUG


* David Woodhouse <dwmw2@...radead.org> wrote:

> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>  arch/x86/kernel/relocate_kernel_64.S | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 67f6853c7abe..ebbd76c9a3e9 100644
> --- a/arch/x86/kernel/relocate_kernel_64.S
> +++ b/arch/x86/kernel/relocate_kernel_64.S
> @@ -14,6 +14,8 @@
>  #include <asm/nospec-branch.h>
>  #include <asm/unwind_hints.h>
>  
> +#define DEBUG
> +
>  /*
>   * Must be relocatable PIC code callable as a C function, in particular
>   * there must be a plain RET and not jump to return thunk.
> @@ -191,6 +193,8 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
>  	pushw	$0xff
>  	lidt	(%rsp)
>  	addq	$10, %rsp
> +
> +	int3
>  #endif /* DEBUG */

That's a really nice piece of debugging code written in assembly, 
combined with the exception handling feature that generates debug 
output to begin with. Epic effort. :-)

Just curious: did you write this code to debug the series, or was there 
some original hair-tearing regression that motivated you? Is there's an 
upstream fix to marvel at and be horrified about in equal measure?

I'd argue that this debugging code probably needs a default-off Kconfig 
option, even with the obvious hard-coded environmental limitations & 
assumptions it has. Could be useful to very early debugging & would 
preserve your effort without it bitrotting too obviously.

Thanks,

	Ingo