linux-kernel - Re: [PATCH v7 7/8] [DO NOT MERGE] x86/kexec: Add int3 in kexec path for testing

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Message-ID: <Z9K3GXSaZUcVr8cb@gmail.com>
Date: Thu, 13 Mar 2025 11:44:41 +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,
	bsz@...zon.de
Subject: Re: [PATCH v7 7/8] [DO NOT MERGE] x86/kexec: Add int3 in kexec path
 for testing


* 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 | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 17d41e6e1a4b..2b7fc59af373 100644
> --- a/arch/x86/kernel/relocate_kernel_64.S
> +++ b/arch/x86/kernel/relocate_kernel_64.S
> @@ -158,7 +158,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
>  	lidt	(%rsp)
>  	addq	$10, %rsp
>  
> -	//int3
> +	int3

So this is all boot-serialized functionality with no SMP concerns 
whatsoever, right?

If yes then we could use something like this:

	static int exception_selftest = 1;

and add the INT3 point:

	int3
.globl after_int3
after_int3:

And do this in the early exception handler:

	...
	
	if (exception_selftest) {
		exception_selftest = 0;

		print_something_warm_and_fuzzy();

		IRET-to-after_int3;
	}

	...

	... regular exception path ...

... but all in assembly or so ;-)

This would make it reasonably certain that the most complex bits of 
this new debuging code are in working order, all the time.

Thanks,

	Ingo