Re: [PATCH v7 06/10] x86/nmi: Add support to handle NMIs with source information

[Sohil Mehta <sohil.mehta@intel.com>]

On 7/7/2025 6:50 AM, Zhuo, Qiuxu wrote:

> Is it possible for the "source_bitmap" to have some non-NMIS_NO_SOURCE bit set 
> while the user registers their NMI handler with the NMIS_NO_SOURCE type? 
> 

IIUC, this is what you are asking:

Someone registers a handler without any source information.
For example, GHES does this.

register_nmi_handler(NMI_LOCAL, ghes_notify_nmi, 0, "ghes", NMIS_NO_SOURCE);

Now, when a GHES NMI shows up, can it have anything other than bit 0
(NMIS_NO_SOURCE) set in the source bitmap?

I believe the answer is no. Unless the GHES implementation or the
hardware has a bug, this should not happen. If this happens, it would
get logged as an unknown NMI in the kernel log.


> If so, we may need to allow the NMI handler with the NMIS_NO_SOURCE type to be 
> invoked unconditionally to ensure no NMIs are lost.
> 

If the kernel can't rely on the accuracy of the source_bitmap, then
NMI-source as a feature starts losing value.

Sohil