Zack's Kernel News

Article from Issue 180/2015

Author(s): Zack Brown

Chronicler Zack Brown reports on the latest news, views, dilemmas, and developments within the Linux kernel community.

NMI Refactor And Simplification

Andy Lutomirski wasn't satisfied with the non-masking interrupt (NMI) system, as implemented in the Linux x86 architecture. An NMI is when the system hardware interrupts the normal flow of code execution in order to perform a task that's so important it can't be ignored or deferred by the operating system. For example, if the system detects bad memory, it might trigger an NMI that would halt the OS.

But even – maybe especially – in such dire circumstances, it's important for the kernel to behave in as orderly a way as possible to preserve user data and leave the system in as consistent a state as possible. One NMI may occur within another, depending on the cause of the interrupts. That nesting has to be handled gracefully, and code sometimes needs to know whether it was called from within another NMI – information that is notoriously tricky to determine. However, Andy reported that in the current code, in certain corner cases, NMIs could nest improperly, malicious user code could cause some NMIs to be ignored entirely, and some code paths simply handled NMIs incorrectly because the kernel code was wrong.

He posted some patches to try to fix things up but quickly found himself plunging down the rabbit's hole of nested NMI call subtlety, as he discovered more about how the existing NMI code behaved. The whole discussion was fascinating.

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