[tip: x86/alternatives] x86/nospec: Shorten RESET_CALL_DEPTH

[tip-bot2 for Peter Zijlstra]

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