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umonitor, umwait, and tpause are a set of user wait instructions. umonitor arms address monitoring hardware using an address. The address range is determined by using CPUID.0x5. A store to an address within the specified address range triggers the monitoring hardware to wake up the processor waiting in umwait. umwait instructs the processor to enter an implementation-dependent optimized state while monitoring a range of addresses. The optimized state may be either a light-weight power/performance optimized state (C0.1 state) or an improved power/performance optimized state (C0.2 state). tpause instructs the processor to enter an implementation-dependent optimized state C0.1 or C0.2 state and wake up when time-stamp counter reaches specified timeout. The three instructions may be executed at any privilege level. The instructions provide power saving method while waiting in user space. Additionally, they can allow a sibling hyperthread to make faster progress while this thread is waiting. One example of an application usage of umwait is when waiting for input data from another application, such as a user level multi-threaded packet processing engine. Availability of the user wait instructions is indicated by the presence of the CPUID feature flag WAITPKG CPUID.0x07.0x0:ECX[5]. Detailed information on the instructions and CPUID feature WAITPKG flag can be found in the latest Intel Architecture Instruction Set Extensions and Future Features Programming Reference and Intel 64 and IA-32 Architectures Software Developer's Manual. 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/1560994438-235698-2-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.