linux-kernel - Re: [PATCH v5 5/9] x86/nmi: Add support to handle NMIs with source information

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Message-ID: <ad44acdb-3e4b-4160-b722-d923e26fd029@intel.com>
Date: Thu, 8 May 2025 17:45:03 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Peter Zijlstra <peterz@...radead.org>, "H. Peter Anvin" <hpa@...or.com>
CC: <x86@...nel.org>, <linux-kernel@...r.kernel.org>, Xin Li <xin@...or.com>,
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Subject: Re: [PATCH v5 5/9] x86/nmi: Add support to handle NMIs with source
 information

On 5/8/2025 1:49 PM, Peter Zijlstra wrote:
> On Thu, May 08, 2025 at 01:23:04PM -0700, H. Peter Anvin wrote:
>> On May 8, 2025 5:15:44 AM PDT, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>>>> Looks good, except when fred_event_data() returns 0. I don't expect it
>>>> to happen in practice. But, maybe with new hardware and eventually
>>>> different hypervisors being involved, it is a possibility.
>>>>
>>>> We can either call it a bug that an NMI happened without source
>>>> information. Or be extra nice and do this:
>>>>
>>>> if (cpu_feature_enabled(X86_FEATURE_NMI_SOURCE) && type == NMI_LOCAL) {
>>>> 	source = fred_event_data(regs);
>>>> 	if (!source || (source & BIT(0)))
>>>> 		source = ~0UL;
>>>> }
>>>
>>> Perhaps also WARN about the !source case?
>>
>> A 0 should be interpreted such that NMI source is not available, e.g.
>> due to a broken hypervisor or similar.
> 
> I'm reading that as an agreement for WARN-ing on 0. We should definitely
> WARN on broken hypervisors and similar.

Yup, will do.