Re: [PATCH 0/2] VMSCAPE optimization for BHI variant

[Pawan Gupta]

On Mon, Sep 29, 2025 at 07:12:03AM +0200, Jack Wang wrote:
> From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> 
> Hi Pawan,
> 
> Thx for the patches, I tested them on our Intel SierraForest machine with
> fio 4k randread/randwrite from guest, qemu virtio-blk, noticed nice
> performance improvement comparing to the default IBPB before exit to
> userspace mitigation. eg with default IBPB mitigation fio gets 204k IOPS,
> with this new Clear BHB before exit to userspace gets 323k IOPS.

Thanks for sharing the results.

I realized the LFENCE in the clear_bhb_long_loop() is not required. The
ring3 transition after the loop should be serializing anyways. Below patch
gets rid of that LFENCE. It should give some performance boost as well.

--- 8< ---
From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Subject: [PATCH] x86/vmscape: Remove LFENCE from BHB clearing long loop

Long loop is used to clear the branch history when switching from a guest
to host userspace. The LFENCE barrier is not required as ring transition
itself acts as a barrier.

Move the prologue, LFENCE and epilogue out of __CLEAR_BHB_LOOP macro to
allow skipping the LFENCE in the long loop variant. Rename the long loop
function to clear_bhb_long_loop_no_barrier() to reflect the change.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
 arch/x86/entry/entry_64.S            | 32 +++++++++++++++++-----------
 arch/x86/include/asm/entry-common.h  |  2 +-
 arch/x86/include/asm/nospec-branch.h |  4 ++--
 3 files changed, 23 insertions(+), 15 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index f5f62af080d8..bb456a3c652e 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1525,10 +1525,6 @@ SYM_CODE_END(rewind_stack_and_make_dead)
  * Target Selection, rather than taking the slowpath via its_return_thunk.
  */
 .macro	__CLEAR_BHB_LOOP outer_loop_count:req, inner_loop_count:req
-	ANNOTATE_NOENDBR
-	push	%rbp
-	mov	%rsp, %rbp
-
 	movl	$\outer_loop_count, %ecx
 	ANNOTATE_INTRA_FUNCTION_CALL
 	call	1f
@@ -1560,10 +1556,7 @@ SYM_CODE_END(rewind_stack_and_make_dead)
 	jnz	1b
 .Lret2_\@:
 	RET
-5:	lfence
-
-	pop	%rbp
-	RET
+5:
 .endm
 
 /*
@@ -1573,7 +1566,15 @@ SYM_CODE_END(rewind_stack_and_make_dead)
  * setting BHI_DIS_S for the guests.
  */
 SYM_FUNC_START(clear_bhb_loop)
+	ANNOTATE_NOENDBR
+	push	%rbp
+	mov	%rsp, %rbp
+
 	__CLEAR_BHB_LOOP 5, 5
+
+	lfence
+	pop	%rbp
+	RET
 SYM_FUNC_END(clear_bhb_loop)
 EXPORT_SYMBOL_GPL(clear_bhb_loop)
 STACK_FRAME_NON_STANDARD(clear_bhb_loop)
@@ -1584,8 +1585,15 @@ STACK_FRAME_NON_STANDARD(clear_bhb_loop)
  * protects the kernel, but to mitigate the guest influence on the host
  * userspace either IBPB or this sequence should be used. See VMSCAPE bug.
  */
-SYM_FUNC_START(clear_bhb_long_loop)
+SYM_FUNC_START(clear_bhb_long_loop_no_barrier)
+	ANNOTATE_NOENDBR
+	push	%rbp
+	mov	%rsp, %rbp
+
 	__CLEAR_BHB_LOOP 12, 7
-SYM_FUNC_END(clear_bhb_long_loop)
-EXPORT_SYMBOL_GPL(clear_bhb_long_loop)
-STACK_FRAME_NON_STANDARD(clear_bhb_long_loop)
+
+	pop	%rbp
+	RET
+SYM_FUNC_END(clear_bhb_long_loop_no_barrier)
+EXPORT_SYMBOL_GPL(clear_bhb_long_loop_no_barrier)
+STACK_FRAME_NON_STANDARD(clear_bhb_long_loop_no_barrier)
diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index b7b9af1b6413..c70454bdd0e3 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -98,7 +98,7 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 		if (cpu_feature_enabled(X86_FEATURE_IBPB_EXIT_TO_USER))
 			indirect_branch_prediction_barrier();
 		else if (cpu_feature_enabled(X86_FEATURE_CLEAR_BHB_EXIT_TO_USER))
-			clear_bhb_long_loop();
+			clear_bhb_long_loop_no_barrier();
 
 		this_cpu_write(x86_pred_flush_pending, false);
 	}
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 32d52f32a5e7..151f5de1a430 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -388,9 +388,9 @@ extern void write_ibpb(void);
 
 #ifdef CONFIG_X86_64
 extern void clear_bhb_loop(void);
-extern void clear_bhb_long_loop(void);
+extern void clear_bhb_long_loop_no_barrier(void);
 #else
-static inline void clear_bhb_long_loop(void) {}
+static inline void clear_bhb_long_loop_no_barrier(void) {}
 #endif
 
 extern void (*x86_return_thunk)(void);