Commit-ID: 2eb9137c8744f9adf1670e9aa52850948a30112b Gitweb: https://git.kernel.org/tip/2eb9137c8744f9adf1670e9aa52850948a30112b Author: Tom Lendacky <[email protected]> AuthorDate: Fri, 12 Jan 2018 19:07:28 -0600 Committer: Ingo Molnar <[email protected]> CommitDate: Sat, 13 Jan 2018 11:28:50 +0100
x86/retpoline: Use LFENCE instead of PAUSE in the retpoline/RSB filling RSB macros 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. Change the use of PAUSE to LFENCE. Signed-off-by: Tom Lendacky <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Borislav Petkov <[email protected]> Cc: Dan Williams <[email protected]> Cc: Dave Hansen <[email protected]> Cc: David Woodhouse <[email protected]> Cc: Greg Kroah-Hartman <[email protected]> Cc: Jiri Kosina <[email protected]> Cc: Josh Poimboeuf <[email protected]> Cc: Kees Cook <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Paul Turner <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Rik van Riel <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Tim Chen <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Ingo Molnar <[email protected]> --- arch/x86/include/asm/nospec-branch.h | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index 402a11c..2c4a09a 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -11,7 +11,7 @@ * Fill the CPU return stack buffer. * * Each entry in the RSB, if used for a speculative 'ret', contains an - * infinite 'pause; jmp' loop to capture speculative execution. + * infinite 'lfence; jmp' loop to capture speculative execution. * * This is required in various cases for retpoline and IBRS-based * mitigations for the Spectre variant 2 vulnerability. Sometimes to @@ -37,12 +37,12 @@ 771: \ call 772f; \ 773: /* speculation trap */ \ - pause; \ + lfence; \ jmp 773b; \ 772: \ call 774f; \ 775: /* speculation trap */ \ - pause; \ + lfence; \ jmp 775b; \ 774: \ dec reg; \ @@ -72,7 +72,7 @@ .macro RETPOLINE_JMP reg:req call .Ldo_rop_\@ .Lspec_trap_\@: - pause + lfence jmp .Lspec_trap_\@ .Ldo_rop_\@: mov \reg, (%_ASM_SP) @@ -164,7 +164,7 @@ " jmp 904f;\n" \ " .align 16\n" \ "901: call 903f;\n" \ - "902: pause;\n" \ + "902: lfence;\n" \ " jmp 902b;\n" \ " .align 16\n" \ "903: addl $4, %%esp;\n" \
