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The PAUSE instruction is currently used in the retpoline and RSB filling macros as a speculation trap. The use of PAUSE was originally suggested because it showed a very, very small difference in the amount of cycles/time used to execute the retpoline as compared to LFENCE. On AMD, the PAUSE instruction is not a serializing instruction, so the pause/jmp loop will use excess power as it is speculated over waiting for return to mispredict to the correct target. The RSB filling macro is applicable to AMD, and, if software is unable to verify that LFENCE is serializing on AMD (possible when running under a hypervisor), the generic retpoline support will be used and, so, is also applicable to AMD. Keep the current usage of PAUSE for Intel, but add an LFENCE instruction to the speculation trap for AMD. The same sequence has been adopted by GCC for the GCC generated retpolines. Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Borislav Petkov <bp@alien8.de> Acked-by: David Woodhouse <dwmw@amazon.co.uk> Acked-by: Arjan van de Ven <arjan@linux.intel.com> Cc: Rik van Riel <riel@redhat.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: Paul Turner <pjt@google.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Jiri Kosina <jikos@kernel.org> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org> Cc: Kees Cook <keescook@google.com> Link: https://lkml.kernel.org/r/20180113232730.31060.36287.stgit@tlendack-t1.amdoffice.net
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.