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When saving xstate to a kernel/user XSAVE area with the XSAVE family of instructions, the current code applies the 'full' instruction mask (-1), which tries to XSAVE all possible features. This method relies on hardware to trim 'all possible' down to what is enabled in the hardware. The code works well for now. However, there will be a problem, if some features are enabled in hardware, but are not suitable to be saved into all kernel XSAVE buffers, like task->fpu, due to performance consideration. One such example is the Last Branch Records (LBR) state. The LBR state only contains valuable information when LBR is explicitly enabled by the perf subsystem, and the size of an LBR state is large (808 bytes for now). To avoid both CPU overhead and space overhead at each context switch, the LBR state should not be saved into task->fpu like other state components. It should be saved/restored on demand when LBR is enabled in the perf subsystem. Current copy_xregs_to_* will trigger a buffer overflow for such cases. Three sites use the '-1' instruction mask which must be updated. Two are saving/restoring the xstate to/from a kernel-allocated XSAVE buffer and can use 'xfeatures_mask_all', which will save/restore all of the features present in a normal task FPU buffer. The last one saves the register state directly to a user buffer. It could also use 'xfeatures_mask_all'. Just as it was with the '-1' argument, any supervisor states in the mask will be filtered out by the hardware and not saved to the buffer. But, to be more explicit about what is expected to be saved, use xfeatures_mask_user() for the instruction mask. KVM includes the header file fpu/internal.h. To avoid 'undefined xfeatures_mask_all' compiling issue, move copy_fpregs_to_fpstate() to fpu/core.c and export it, because: - The xfeatures_mask_all is indirectly used via copy_fpregs_to_fpstate() by KVM. The function which is directly used by other modules should be exported. - The copy_fpregs_to_fpstate() is a function, while xfeatures_mask_all is a variable for the "internal" FPU state. It's safer to export a function than a variable, which may be implicitly changed by others. - The copy_fpregs_to_fpstate() is a big function with many checks. The removal of the inline keyword should not impact the performance. Signed-off-by: Kan Liang <kan.liang@linux.intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Dave Hansen <dave.hansen@intel.com> Link: https://lkml.kernel.org/r/1593780569-62993-20-git-send-email-kan.liang@linux.intel.com |
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| README | ||
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.