The following commit has been merged into the x86/alternatives branch of tip: Commit-ID: 3496d1c64a0fcc9bae3ed40decc3ecd7f8ac072f Gitweb: https://git.kernel.org/tip/3496d1c64a0fcc9bae3ed40decc3ecd7f8ac072f Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx> AuthorDate: Fri, 10 Feb 2023 10:10:57 Committer: Borislav Petkov (AMD) <bp@xxxxxxxxx> CommitterDate: Wed, 31 May 2023 13:40:57 +02:00 x86/nospec: Shorten RESET_CALL_DEPTH RESET_CALL_DEPTH is a pretty fat monster and blows up UNTRAIN_RET to 20 bytes: 19: 48 c7 c0 80 00 00 00 mov $0x80,%rax 20: 48 c1 e0 38 shl $0x38,%rax 24: 65 48 89 04 25 00 00 00 00 mov %rax,%gs:0x0 29: R_X86_64_32S pcpu_hot+0x10 Shrink it by 4 bytes: 0: 31 c0 xor %eax,%eax 2: 48 0f ba e8 3f bts $0x3f,%rax 7: 65 48 89 04 25 00 00 00 00 mov %rax,%gs:0x0 Shrink RESET_CALL_DEPTH_FROM_CALL by 5 bytes by only setting %al, the other bits are shifted out (the same could be done for RESET_CALL_DEPTH, but the XOR+BTS sequence has less dependencies due to the zeroing). Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx> Link: https://lore.kernel.org/r/20230515093020.729622326@xxxxxxxxxxxxx --- arch/x86/include/asm/nospec-branch.h | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index edb2b0c..55388c9 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -84,12 +84,12 @@ movq $-1, PER_CPU_VAR(pcpu_hot + X86_call_depth); #define RESET_CALL_DEPTH \ - mov $0x80, %rax; \ - shl $56, %rax; \ + xor %eax, %eax; \ + bts $63, %rax; \ movq %rax, PER_CPU_VAR(pcpu_hot + X86_call_depth); #define RESET_CALL_DEPTH_FROM_CALL \ - mov $0xfc, %rax; \ + movb $0xfc, %al; \ shl $56, %rax; \ movq %rax, PER_CPU_VAR(pcpu_hot + X86_call_depth); \ CALL_THUNKS_DEBUG_INC_CALLS
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