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Reduce system call overhead time (round trip time for invoking a non-existent system call) by 25%. With the removal of set_fs() [1] lazy control register handling was removed in order to keep kernel entry and exit simple. However this made system calls slower. With the conversion to generic entry [2] and numerous follow up changes which simplified the entry code significantly, adding support for lazy asce handling doesn't add much complexity to the entry code anymore. In particular this means: - On kernel entry the primary asce is not modified and contains the user asce - Kernel accesses which require secondary-space mode (for example futex operations) are surrounded by enable_sacf_uaccess() and disable_sacf_uaccess() calls. enable_sacf_uaccess() sets the primary asce to kernel asce so that the sacf instruction can be used to switch to secondary-space mode. The primary asce is changed back to user asce with disable_sacf_uaccess(). The state of the control register which contains the primary asce is reflected with a new TIF_ASCE_PRIMARY bit. This is required on context switch so that the correct asce is restored for the scheduled in process. In result address spaces are now setup like this: CPU running in | %cr1 ASCE | %cr7 ASCE | %cr13 ASCE -----------------------------|-----------|-----------|----------- user space | user | user | kernel kernel (no sacf) | user | user | kernel kernel (during sacf uaccess) | kernel | user | kernel kernel (kvm guest execution) | guest | user | kernel In result cr1 control register content is not changed except for: - futex system calls - legacy s390 PCI system calls - the kvm specific cmpxchg_user_key() uaccess helper This leads to faster system call execution. [1]87d5986345("s390/mm: remove set_fs / rework address space handling") [2]56e62a7370("s390: convert to generic entry") Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com> Signed-off-by: Heiko Carstens <hca@linux.ibm.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 reStructuredText 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.