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: <0CDE8ED7-D8D2-4053-AE4B-4CFF59DB56F7@infradead.org>
Date: Thu, 13 Mar 2025 12:06:58 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Ingo Molnar <mingo@...nel.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

On 13 March 2025 11:44:41 CET, Ingo Molnar <mingo@...nel.org> wrote:
>
>* 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

The exception handler already returns if the exception was int3, but not for anything else. Less so the "print something warm and fuzzy" part; it just does the same register dump. But we could change that.

I'm less keen on making it unconditional though. Kexec is a performance-critical path when every millisecond is perceived as guest steal time, and the serial output should only happen in production if something goes *wrong*.

And besides, most kexec users don't have early_printk enabled anyway so if we break them, this idea doesn't help.