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commit 7eb314a22800457396f541c655697dabd71e44a7 upstream stop_machine() does not prevent the spin-waiting sibling from handling an NMI, which is obviously violating the whole concept of rendezvous. Implement a static branch right in the beginning of the NMI handler which is nopped out except when enabled by the late loading mechanism. The late loader enables the static branch before stop_machine() is invoked. Each CPU has an nmi_enable in its control structure which indicates whether the CPU should go into the update routine. This is required to bridge the gap between enabling the branch and actually being at the point where it is required to enter the loader wait loop. Each CPU which arrives in the stopper thread function sets that flag and issues a self NMI right after that. If the NMI function sees the flag clear, it returns. If it's set it clears the flag and enters the rendezvous. This is safe against a real NMI which hits in between setting the flag and sending the NMI to itself. The real NMI will be swallowed by the microcode update and the self NMI will then let stuff continue. Otherwise this would end up with a spurious NMI. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> Link: https://lore.kernel.org/r/20231002115903.489900814@linutronix.de Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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.