linux-kernel - Re: [PATCH v2] x86/nmi: Add an emergency handler in nmi_desc & use it in nmi_shootdown

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Message-ID: <59b254dc-acf6-4114-b6b4-a7ae517bfa06@redhat.com>
Date: Wed, 4 Dec 2024 14:28:35 -0500
From: Waiman Long <llong@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>, Waiman Long <llong@...hat.com>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
 "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] x86/nmi: Add an emergency handler in nmi_desc & use it
 in nmi_shootdown_cpus()

On 12/4/24 1:03 PM, Thomas Gleixner wrote:
> On Wed, Dec 04 2024 at 12:23, Waiman Long wrote:
>> On 12/4/24 8:10 AM, Thomas Gleixner wrote:
>>> On Tue, Dec 03 2024 at 10:07, Waiman Long wrote:
>>>> +	/*
>>>> +	 * Call the emergency handler first, if set
>>>> +	 * Emergency handler is not traced or checked by nmi_check_duration().
>>>> +	 */
>>>> +	ehandler = READ_ONCE(desc->emerg_handler);
>>>> +	if (ehandler)
>>>> +		handled = ehandler(type, regs);
>>> Shouldn't this just stop processing right here?
>> Yes in the case of crash_nmi_callback(). I suppose it is a no-return
>> call. As the emergency handler is supposed to be a general mechanism in
>> design, I don't want to make too many assumptions of what will happen
>> when the handler is invoked.
> I'm not convinced that this should be used as a general mechanism. It's
> for emergency situations and that's where it stops. If the thing
> returns, it's a bug IMO.

OK, I am fine with that. I will put a BUG_ON() after that in the next 
version.

Thanks,
Longman