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The signal return fast path directly restores user states from the user buffer. Once that succeeds, restore supervisor states (but only when they are not yet restored). For the slow path, save supervisor states to preserve them across context switches, and restore after the user states are restored. The previous version has the overhead of an XSAVES in both the fast and the slow paths. It is addressed as the following: - In the fast path, only do an XRSTORS. - In the slow path, do a supervisor-state-only XSAVES, and relocate the buffer contents. Some thoughts in the implementation: - In the slow path, can any supervisor state become stale between save/restore? Answer: set_thread_flag(TIF_NEED_FPU_LOAD) protects the xstate buffer. - In the slow path, can any code reference a stale supervisor state register between save/restore? Answer: In the current lazy-restore scheme, any reference to xstate registers needs fpregs_lock()/fpregs_unlock() and __fpregs_load_activate(). - Are there other options? One other option is eagerly restoring all supervisor states. Currently, CET user-mode states and ENQCMD's PASID do not need to be eagerly restored. The upcoming CET kernel-mode states (24 bytes) need to be eagerly restored. To me, eagerly restoring all supervisor states adds more overhead then benefit at this point. Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com> Signed-off-by: Borislav Petkov <bp@suse.de> Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com> Link: https://lkml.kernel.org/r/20200512145444.15483-11-yu-cheng.yu@intel.com |
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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.