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umwait or tpause allows the processor to enter a light-weight power/performance optimized state (C0.1 state) or an improved power/performance optimized state (C0.2 state) for a period specified by the instruction or until the system time limit or until a store to the monitored address range in umwait. IA32_UMWAIT_CONTROL MSR register allows the OS to enable/disable C0.2 on the processor and to set the maximum time the processor can reside in C0.1 or C0.2. By default C0.2 is enabled so the user wait instructions can enter the C0.2 state to save more power with slower wakeup time. Andy Lutomirski proposed to set the maximum umwait time to 100000 cycles by default. A quote from Andy: "What I want to avoid is the case where it works dramatically differently on NO_HZ_FULL systems as compared to everything else. Also, UMWAIT may behave a bit differently if the max timeout is hit, and I'd like that path to get exercised widely by making it happen even on default configs." A sysfs interface to adjust the time and the C0.2 enablement is provided in a follow up change. [ tglx: Renamed MSR_IA32_UMWAIT_CONTROL_MAX_TIME to MSR_IA32_UMWAIT_CONTROL_TIME_MASK because the constant is used as mask throughout the code. Massaged comments and changelog ] Signed-off-by: Fenghua Yu <fenghua.yu@intel.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Ashok Raj <ashok.raj@intel.com> Reviewed-by: Andy Lutomirski <luto@kernel.org> Cc: "Borislav Petkov" <bp@alien8.de> Cc: "H Peter Anvin" <hpa@zytor.com> Cc: "Peter Zijlstra" <peterz@infradead.org> Cc: "Tony Luck" <tony.luck@intel.com> Cc: "Ravi V Shankar" <ravi.v.shankar@intel.com> Link: https://lkml.kernel.org/r/1560994438-235698-3-git-send-email-fenghua.yu@intel.com
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.