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The IPI shorthand functionality delivers IPI/NMI broadcasts to all CPUs in the system. This can have similar side effects as the MCE broadcasting when CPUs are waiting in the BIOS or are offlined. The kernel tracks already the state of offlined CPUs whether they have been brought up at least once so that the CR4 MCE bit is set to make sure that MCE broadcasts can't brick the machine. Utilize that information and compare it to the cpu_present_mask. If all present CPUs have been brought up at least once then the broadcast side effect is mitigated by disabling regular interrupt/IPI delivery in the APIC itself and by the cpu offline check at the begin of the NMI handler. Use a static key to switch between broadcasting via shorthands or sending the IPI/NMI one by one. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20190722105220.386410643@linutronix.de
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.