On Fri, 2018-01-12 at 19:07 -0600, Tom Lendacky wrote: > 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 <thomas.lenda...@amd.com>
Conditionally-Acked-by: David Woodhouse <d...@amazon.co.uk> The condition being, as noted, that I'd really like to see it acked by Arjan/Asit and Paul. > --- > 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" \ > >
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