[PATCH v10 00/12] VMSCAPE optimization for BHI variant

[PATCH v10 00/12] VMSCAPE optimization for BHI variant

[Pawan Gupta]

v10:
- Add patches to define EXPORT_STATIC_CALL_FOR_MODULES() and
  EXPORT_STATIC_CALL_FOR_KVM(), so that vmscape_predictor_flush static key
  is only accessible to KVM and not to other kernel modules. (PeterZ)
  (Borisov earlier objected to exporting the static key to all modules, but
  now the static key is only exported to KVM. I guess that resolves the
  concern.)
- Avoid an explicit call to vmscape_mitigation_enabled() and instead use
  static_call_query() in VMexit hot path. (Sean)
- Drop vmscape_mitigation_enabled(), as it is no longer needed.
- Rebased to v7.0

v9: https://lore.kernel.org/r/20260402-vmscape-bhb-v9-0-94d16bc29774@linux.intel.com
- Use global variables for BHB loop counters instead of ALTERNATIVE-based
  approach. (Dave & others)
- Use 32-bit registers (%eax/%ecx) for loop counters, loaded via movzbl
  from 8-bit globals. 8-bit registers (e.g. %ah in the inner loop) caused
  performance regression on certain CPUs due to partial-register stalls. (David Laight)
- Let BPF save/restore %rax/%rcx as in the original implementation, since
  it is the only caller that needs these registers preserved across the
  BHB clearing sequence.
- Drop Reviewed-by from patch 2/10 as the implementation changed significantly.
- Apply Tested-by from Jon Kohler to the series (except patch 2/10).
- Fix commit message grammar. (Borislav)
- Rebased to v7.0-rc6.

v8: https://lore.kernel.org/r/20260324-vmscape-bhb-v8-0-68bb524b3ab9@linux.intel.com
- Use helper in KVM to convey the mitigation status. (PeterZ/Borisov)
- Fix the documentation for default vmscape mitigation. (BPF bot)
- Remove the stray lines in bug.c (BPF bot).
- Updated commit messages and comments.
- Rebased to v7.0-rc5.

v7: https://lore.kernel.org/r/20260319-vmscape-bhb-v7-0-b76a777a98af@linux.intel.com
- s/This allows/Allow/ and s/This does adds/This adds/ in patch 1/10 commit
  message (Borislav).
- Minimize register usage in BHB clearing seq. (David Laight)
  - Instead of separate ecx/eax counters, use al/ah.
  - Adjust the alignment of RET due to register size change.
  - save/restore rax in the seq itself.
  - Remove the save/restore of rax/rcx for BPF callers.
- Rename clear_bhb_loop() to clear_bhb_loop_nofence() to make it
  obvious that the LFENCE is not part of the sequence (Borislav).
- Fix Kconfig: s/select/depends on/ HAVE_STATIC_CALL (PeterZ).
- Rebased to v7.0-rc4.

v6: https://lore.kernel.org/r/20251201-vmscape-bhb-v6-0-d610dd515714@linux.intel.com
- Remove semicolon at the end of asm in ALTERNATIVE (Uros).
- Fix build warning in vmscape_select_mitigation() (LKP).
- Rebased to v6.18.

v5: https://lore.kernel.org/r/20251126-vmscape-bhb-v5-2-02d66e423b00@linux.intel.com
- For BHI seq, limit runtime-patching to loop counts only (Dave).
  Dropped 2 patches that moved the BHB seq to a macro.
- Remove redundant switch cases in vmscape_select_mitigation() (Nikolay).
- Improve commit message (Nikolay).
- Collected tags.

v4: https://lore.kernel.org/r/20251119-vmscape-bhb-v4-0-1adad4e69ddc@linux.intel.com
- Move LFENCE to the callsite, out of clear_bhb_loop(). (Dave)
- Make clear_bhb_loop() work for larger BHB. (Dave)
  This now uses hardware enumeration to determine the BHB size to clear.
- Use write_ibpb() instead of indirect_branch_prediction_barrier() when
  IBPB is known to be available. (Dave)
- Use static_call() to simplify mitigation at exit-to-userspace. (Dave)
- Refactor vmscape_select_mitigation(). (Dave)
- Fix vmscape=on which was wrongly behaving as AUTO. (Dave)
- Split the patches. (Dave)
  - Patch 1-4 prepares for making the sequence flexible for VMSCAPE use.
  - Patch 5 trivial rename of variable.
  - Patch 6-8 prepares for deploying BHB mitigation for VMSCAPE.
  - Patch 9 deploys the mitigation.
  - Patch 10-11 fixes ON Vs AUTO mode.

v3: https://lore.kernel.org/r/20251027-vmscape-bhb-v3-0-5793c2534e93@linux.intel.com
- s/x86_pred_flush_pending/x86_predictor_flush_exit_to_user/ (Sean).
- Removed IBPB & BHB-clear mutual exclusion at exit-to-userspace.
- Collected tags.

v2: https://lore.kernel.org/r/20251015-vmscape-bhb-v2-0-91cbdd9c3a96@linux.intel.com
- Added check for IBPB feature in vmscape_select_mitigation(). (David)
- s/vmscape=auto/vmscape=on/ (David)
- Added patch to remove LFENCE from VMSCAPE BHB-clear sequence.
- Rebased to v6.18-rc1.

v1: https://lore.kernel.org/r/20250924-vmscape-bhb-v1-0-da51f0e1934d@linux.intel.com

Hi All,

These patches aim to improve the performance of a recent mitigation for
VMSCAPE[1] vulnerability. This improvement is relevant for BHI variant of
VMSCAPE that affect Alder Lake and newer processors.

The current mitigation approach uses IBPB on kvm-exit-to-userspace for all
affected range of CPUs. This is an overkill for CPUs that are only affected
by the BHI variant. On such CPUs clearing the branch history is sufficient
for VMSCAPE, and also more apt as the underlying issue is due to poisoned
branch history.

Below is the iPerf data for transfer between guest and host, comparing IBPB
and BHB-clear mitigation. BHB-clear shows performance improvement over IBPB
in most cases.

Platform: Emerald Rapids
Baseline: vmscape=off
Target: IBPB at VMexit-to-userspace Vs the new BHB-clear at
	VMexit-to-userspace mitigation (both compared against baseline).

(pN = N parallel connections)

| iPerf user-net | IBPB    | BHB Clear |
|----------------|---------|-----------|
| UDP 1-vCPU_p1  | -12.5%  |   1.3%    |
| TCP 1-vCPU_p1  | -10.4%  |  -1.5%    |
| TCP 1-vCPU_p1  | -7.5%   |  -3.0%    |
| UDP 4-vCPU_p16 | -3.7%   |  -3.7%    |
| TCP 4-vCPU_p4  | -2.9%   |  -1.4%    |
| UDP 4-vCPU_p4  | -0.6%   |   0.0%    |
| TCP 4-vCPU_p4  |  3.5%   |   0.0%    |

| iPerf bridge-net | IBPB    | BHB Clear |
|------------------|---------|-----------|
| UDP 1-vCPU_p1    | -9.4%   |  -0.4%    |
| TCP 1-vCPU_p1    | -3.9%   |  -0.5%    |
| UDP 4-vCPU_p16   | -2.2%   |  -3.8%    |
| TCP 4-vCPU_p4    | -1.0%   |  -1.0%    |
| TCP 4-vCPU_p4    |  0.5%   |   0.5%    |
| UDP 4-vCPU_p4    |  0.0%   |   0.9%    |
| TCP 1-vCPU_p1    |  0.0%   |   0.9%    |

| iPerf vhost-net | IBPB    | BHB Clear |
|-----------------|---------|-----------|
| UDP 1-vCPU_p1   | -4.3%   |   1.0%    |
| TCP 1-vCPU_p1   | -3.8%   |  -0.5%    |
| TCP 1-vCPU_p1   | -2.7%   |  -0.7%    |
| UDP 4-vCPU_p16  | -0.7%   |  -2.2%    |
| TCP 4-vCPU_p4   | -0.4%   |   0.8%    |
| UDP 4-vCPU_p4   |  0.4%   |  -0.7%    |
| TCP 4-vCPU_p4   |  0.0%   |   0.6%    |

[1] https://comsec.ethz.ch/research/microarch/vmscape-exposing-and-exploiting-incomplete-branch-predictor-isolation-in-cloud-environments/

---
Pawan Gupta (12):
      x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb_loop()
      x86/bhi: Make clear_bhb_loop() effective on newer CPUs
      x86/bhi: Rename clear_bhb_loop() to clear_bhb_loop_nofence()
      x86/vmscape: Rename x86_ibpb_exit_to_user to x86_predictor_flush_exit_to_user
      x86/vmscape: Move mitigation selection to a switch()
      x86/vmscape: Use write_ibpb() instead of indirect_branch_prediction_barrier()
      static_call: Add EXPORT_STATIC_CALL_FOR_MODULES()
      kvm: Define EXPORT_STATIC_CALL_FOR_KVM()
      x86/vmscape: Use static_call() for predictor flush
      x86/vmscape: Deploy BHB clearing mitigation
      x86/vmscape: Resolve conflict between attack-vectors and vmscape=force
      x86/vmscape: Add cmdline vmscape=on to override attack vector controls

 Documentation/admin-guide/hw-vuln/vmscape.rst   | 15 ++++-
 Documentation/admin-guide/kernel-parameters.txt |  6 +-
 arch/x86/Kconfig                                |  1 +
 arch/x86/entry/entry_64.S                       | 21 ++++---
 arch/x86/include/asm/cpufeatures.h              |  2 +-
 arch/x86/include/asm/entry-common.h             | 13 ++--
 arch/x86/include/asm/kvm_types.h                |  1 +
 arch/x86/include/asm/nospec-branch.h            | 15 +++--
 arch/x86/kernel/cpu/bugs.c                      | 84 +++++++++++++++++++++----
 arch/x86/kvm/x86.c                              |  4 +-
 arch/x86/net/bpf_jit_comp.c                     |  4 +-
 include/linux/kvm_types.h                       | 13 +++-
 include/linux/static_call.h                     |  8 +++
 13 files changed, 150 insertions(+), 37 deletions(-)
---
base-commit: 028ef9c96e96197026887c0f092424679298aae8
change-id: 20250916-vmscape-bhb-d7d469977f2f

Best regards,
--  
Thanks,
Pawan

[PATCH v10 01/12] x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb_loop()

[Pawan Gupta]

Currently, the BHB clearing sequence is followed by an LFENCE to prevent
transient execution of subsequent indirect branches prematurely. However,
the LFENCE barrier could be unnecessary in certain cases. For example, when
the kernel is using the BHI_DIS_S mitigation, and BHB clearing is only
needed for userspace. In such cases, the LFENCE is redundant because ring
transitions would provide the necessary serialization.

Below is a quick recap of BHI mitigation options:

On Alder Lake and newer

    BHI_DIS_S: Hardware control to mitigate BHI in ring0. This has low
    performance overhead.

    Long loop: Alternatively, a longer version of the BHB clearing sequence
    can be used to mitigate BHI. It can also be used to mitigate the BHI
    variant of VMSCAPE. This is not yet implemented in Linux.

On older CPUs

    Short loop: Clears BHB at kernel entry and VMexit. The "Long loop" is
    effective on older CPUs as well, but should be avoided because of
    unnecessary overhead.

On Alder Lake and newer CPUs, eIBRS isolates the indirect targets between
guest and host. But when affected by the BHI variant of VMSCAPE, a guest's
branch history may still influence indirect branches in userspace. This
also means the big hammer IBPB could be replaced with a cheaper option that
clears the BHB at exit-to-userspace after a VMexit.

In preparation for adding the support for the BHB sequence (without LFENCE)
on newer CPUs, move the LFENCE to the caller side after clear_bhb_loop() is
executed. Allow callers to decide whether they need the LFENCE or not. This
adds a few extra bytes to the call sites, but it obviates the need for
multiple variants of clear_bhb_loop().

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Jon Kohler <jon@nutanix.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/entry/entry_64.S            | 5 ++++-
 arch/x86/include/asm/nospec-branch.h | 4 ++--
 arch/x86/net/bpf_jit_comp.c          | 2 ++
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 42447b1e1dff..3a180a36ca0e 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1528,6 +1528,9 @@ SYM_CODE_END(rewind_stack_and_make_dead)
  * refactored in the future if needed. The .skips are for safety, to ensure
  * that all RETs are in the second half of a cacheline to mitigate Indirect
  * Target Selection, rather than taking the slowpath via its_return_thunk.
+ *
+ * Note, callers should use a speculation barrier like LFENCE immediately after
+ * a call to this function to ensure BHB is cleared before indirect branches.
  */
 SYM_FUNC_START(clear_bhb_loop)
 	ANNOTATE_NOENDBR
@@ -1562,7 +1565,7 @@ SYM_FUNC_START(clear_bhb_loop)
 	sub	$1, %ecx
 	jnz	1b
 .Lret2:	RET
-5:	lfence
+5:
 	pop	%rbp
 	RET
 SYM_FUNC_END(clear_bhb_loop)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 4f4b5e8a1574..70b377fcbc1c 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -331,11 +331,11 @@
 
 #ifdef CONFIG_X86_64
 .macro CLEAR_BRANCH_HISTORY
-	ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP
+	ALTERNATIVE "", "call clear_bhb_loop; lfence", X86_FEATURE_CLEAR_BHB_LOOP
 .endm
 
 .macro CLEAR_BRANCH_HISTORY_VMEXIT
-	ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_VMEXIT
+	ALTERNATIVE "", "call clear_bhb_loop; lfence", X86_FEATURE_CLEAR_BHB_VMEXIT
 .endm
 #else
 #define CLEAR_BRANCH_HISTORY
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index e9b78040d703..63d6c9fa5e80 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1624,6 +1624,8 @@ static int emit_spectre_bhb_barrier(u8 **pprog, u8 *ip,
 
 		if (emit_call(&prog, func, ip))
 			return -EINVAL;
+		/* Don't speculate past this until BHB is cleared */
+		EMIT_LFENCE();
 		EMIT1(0x59); /* pop rcx */
 		EMIT1(0x58); /* pop rax */
 	}

-- 
2.34.1

Re: [PATCH v10 01/12] x86/bhi: x86/vmscape: Move LFENCE out of clear_bhb_loop()

[Pawan Gupta]

On Tue, Apr 14, 2026 at 12:05:28AM -0700, Pawan Gupta wrote:
> Currently, the BHB clearing sequence is followed by an LFENCE to prevent
> transient execution of subsequent indirect branches prematurely. However,
> the LFENCE barrier could be unnecessary in certain cases. For example, when
> the kernel is using the BHI_DIS_S mitigation, and BHB clearing is only
> needed for userspace. In such cases, the LFENCE is redundant because ring
> transitions would provide the necessary serialization.
> 
> Below is a quick recap of BHI mitigation options:
> 
> On Alder Lake and newer
> 
>     BHI_DIS_S: Hardware control to mitigate BHI in ring0. This has low
>     performance overhead.
> 
>     Long loop: Alternatively, a longer version of the BHB clearing sequence
>     can be used to mitigate BHI. It can also be used to mitigate the BHI
>     variant of VMSCAPE. This is not yet implemented in Linux.
> 
> On older CPUs
> 
>     Short loop: Clears BHB at kernel entry and VMexit. The "Long loop" is
>     effective on older CPUs as well, but should be avoided because of
>     unnecessary overhead.
> 
> On Alder Lake and newer CPUs, eIBRS isolates the indirect targets between
> guest and host. But when affected by the BHI variant of VMSCAPE, a guest's
> branch history may still influence indirect branches in userspace. This
> also means the big hammer IBPB could be replaced with a cheaper option that
> clears the BHB at exit-to-userspace after a VMexit.
> 
> In preparation for adding the support for the BHB sequence (without LFENCE)
> on newer CPUs, move the LFENCE to the caller side after clear_bhb_loop() is
> executed. Allow callers to decide whether they need the LFENCE or not. This
> adds a few extra bytes to the call sites, but it obviates the need for
> multiple variants of clear_bhb_loop().
> 
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Tested-by: Jon Kohler <jon@nutanix.com>
> Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> ---

Sorry this is missing Boris's Ack, I will fix.

> Acked-by: Borislav Petkov (AMD) <bp@alien8.de>

[PATCH v10 02/12] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

[Pawan Gupta]

As a mitigation for BHI, clear_bhb_loop() executes branches that overwrite
the Branch History Buffer (BHB). On Alder Lake and newer parts this
sequence is not sufficient because it doesn't clear enough entries. This
was not an issue because these CPUs use the BHI_DIS_S hardware mitigation
in the kernel.

Now with VMSCAPE (BHI variant) it is also required to isolate branch
history between guests and userspace. Since BHI_DIS_S only protects the
kernel, the newer CPUs also use IBPB.

A cheaper alternative to the current IBPB mitigation is clear_bhb_loop().
But it currently does not clear enough BHB entries to be effective on newer
CPUs with larger BHB. At boot, dynamically set the loop count of
clear_bhb_loop() such that it is effective on newer CPUs too.

Introduce global loop counts, initializing them with appropriate value
based on the hardware feature X86_FEATURE_BHI_CTRL.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/entry/entry_64.S            |  8 +++++---
 arch/x86/include/asm/nospec-branch.h |  2 ++
 arch/x86/kernel/cpu/bugs.c           | 13 +++++++++++++
 3 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 3a180a36ca0e..bbd4b1c7ec04 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1536,7 +1536,9 @@ SYM_FUNC_START(clear_bhb_loop)
 	ANNOTATE_NOENDBR
 	push	%rbp
 	mov	%rsp, %rbp
-	movl	$5, %ecx
+
+	movzbl    bhb_seq_outer_loop(%rip), %ecx
+
 	ANNOTATE_INTRA_FUNCTION_CALL
 	call	1f
 	jmp	5f
@@ -1556,8 +1558,8 @@ SYM_FUNC_START(clear_bhb_loop)
 	 * This should be ideally be: .skip 32 - (.Lret2 - 2f), 0xcc
 	 * but some Clang versions (e.g. 18) don't like this.
 	 */
-	.skip 32 - 18, 0xcc
-2:	movl	$5, %eax
+	.skip 32 - 20, 0xcc
+2:	movzbl  bhb_seq_inner_loop(%rip), %eax
 3:	jmp	4f
 	nop
 4:	sub	$1, %eax
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 70b377fcbc1c..87b83ae7c97f 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -548,6 +548,8 @@ DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
 extern void update_spec_ctrl_cond(u64 val);
 extern u64 spec_ctrl_current(void);
 
+extern u8 bhb_seq_inner_loop, bhb_seq_outer_loop;
+
 /*
  * With retpoline, we must use IBRS to restrict branch prediction
  * before calling into firmware.
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 83f51cab0b1e..2cb4a96247d8 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2047,6 +2047,10 @@ enum bhi_mitigations {
 static enum bhi_mitigations bhi_mitigation __ro_after_init =
 	IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI) ? BHI_MITIGATION_AUTO : BHI_MITIGATION_OFF;
 
+/* Default to short BHB sequence values */
+u8 bhb_seq_outer_loop __ro_after_init = 5;
+u8 bhb_seq_inner_loop __ro_after_init = 5;
+
 static int __init spectre_bhi_parse_cmdline(char *str)
 {
 	if (!str)
@@ -3242,6 +3246,15 @@ void __init cpu_select_mitigations(void)
 		x86_spec_ctrl_base &= ~SPEC_CTRL_MITIGATIONS_MASK;
 	}
 
+	/*
+	 * Switch to long BHB clear sequence on newer CPUs (with BHI_CTRL
+	 * support), see Intel's BHI guidance.
+	 */
+	if (cpu_feature_enabled(X86_FEATURE_BHI_CTRL)) {
+		bhb_seq_outer_loop = 12;
+		bhb_seq_inner_loop = 7;
+	}
+
 	x86_arch_cap_msr = x86_read_arch_cap_msr();
 
 	cpu_print_attack_vectors();

-- 
2.34.1

[PATCH v10 03/12] x86/bhi: Rename clear_bhb_loop() to clear_bhb_loop_nofence()

[Pawan Gupta]

To reflect the recent change that moved LFENCE to the caller side.

Suggested-by: Borislav Petkov <bp@alien8.de>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Tested-by: Jon Kohler <jon@nutanix.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/entry/entry_64.S            | 8 ++++----
 arch/x86/include/asm/nospec-branch.h | 6 +++---
 arch/x86/net/bpf_jit_comp.c          | 2 +-
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index bbd4b1c7ec04..1f56d086d312 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1532,7 +1532,7 @@ SYM_CODE_END(rewind_stack_and_make_dead)
  * Note, callers should use a speculation barrier like LFENCE immediately after
  * a call to this function to ensure BHB is cleared before indirect branches.
  */
-SYM_FUNC_START(clear_bhb_loop)
+SYM_FUNC_START(clear_bhb_loop_nofence)
 	ANNOTATE_NOENDBR
 	push	%rbp
 	mov	%rsp, %rbp
@@ -1570,6 +1570,6 @@ SYM_FUNC_START(clear_bhb_loop)
 5:
 	pop	%rbp
 	RET
-SYM_FUNC_END(clear_bhb_loop)
-EXPORT_SYMBOL_FOR_KVM(clear_bhb_loop)
-STACK_FRAME_NON_STANDARD(clear_bhb_loop)
+SYM_FUNC_END(clear_bhb_loop_nofence)
+EXPORT_SYMBOL_FOR_KVM(clear_bhb_loop_nofence)
+STACK_FRAME_NON_STANDARD(clear_bhb_loop_nofence)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 87b83ae7c97f..157eb69c7f0f 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -331,11 +331,11 @@
 
 #ifdef CONFIG_X86_64
 .macro CLEAR_BRANCH_HISTORY
-	ALTERNATIVE "", "call clear_bhb_loop; lfence", X86_FEATURE_CLEAR_BHB_LOOP
+	ALTERNATIVE "", "call clear_bhb_loop_nofence; lfence", X86_FEATURE_CLEAR_BHB_LOOP
 .endm
 
 .macro CLEAR_BRANCH_HISTORY_VMEXIT
-	ALTERNATIVE "", "call clear_bhb_loop; lfence", X86_FEATURE_CLEAR_BHB_VMEXIT
+	ALTERNATIVE "", "call clear_bhb_loop_nofence; lfence", X86_FEATURE_CLEAR_BHB_VMEXIT
 .endm
 #else
 #define CLEAR_BRANCH_HISTORY
@@ -389,7 +389,7 @@ extern void entry_untrain_ret(void);
 extern void write_ibpb(void);
 
 #ifdef CONFIG_X86_64
-extern void clear_bhb_loop(void);
+extern void clear_bhb_loop_nofence(void);
 #endif
 
 extern void (*x86_return_thunk)(void);
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 63d6c9fa5e80..f40e88f87273 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1619,7 +1619,7 @@ static int emit_spectre_bhb_barrier(u8 **pprog, u8 *ip,
 		EMIT1(0x51); /* push rcx */
 		ip += 2;
 
-		func = (u8 *)clear_bhb_loop;
+		func = (u8 *)clear_bhb_loop_nofence;
 		ip += x86_call_depth_emit_accounting(&prog, func, ip);
 
 		if (emit_call(&prog, func, ip))

-- 
2.34.1

[PATCH v10 04/12] x86/vmscape: Rename x86_ibpb_exit_to_user to x86_predictor_flush_exit_to_user

[Pawan Gupta]

With the upcoming changes x86_ibpb_exit_to_user will also be used when BHB
clearing sequence is used. Rename it cover both the cases.

No functional change.

Suggested-by: Sean Christopherson <seanjc@google.com>
Tested-by: Jon Kohler <jon@nutanix.com>
Acked-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/include/asm/entry-common.h  | 6 +++---
 arch/x86/include/asm/nospec-branch.h | 2 +-
 arch/x86/kernel/cpu/bugs.c           | 4 ++--
 arch/x86/kvm/x86.c                   | 2 +-
 4 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index ce3eb6d5fdf9..c45858db16c9 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -94,11 +94,11 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 	 */
 	choose_random_kstack_offset(rdtsc());
 
-	/* Avoid unnecessary reads of 'x86_ibpb_exit_to_user' */
+	/* Avoid unnecessary reads of 'x86_predictor_flush_exit_to_user' */
 	if (cpu_feature_enabled(X86_FEATURE_IBPB_EXIT_TO_USER) &&
-	    this_cpu_read(x86_ibpb_exit_to_user)) {
+	    this_cpu_read(x86_predictor_flush_exit_to_user)) {
 		indirect_branch_prediction_barrier();
-		this_cpu_write(x86_ibpb_exit_to_user, false);
+		this_cpu_write(x86_predictor_flush_exit_to_user, false);
 	}
 }
 #define arch_exit_to_user_mode_prepare arch_exit_to_user_mode_prepare
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 157eb69c7f0f..0381db59c39d 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -533,7 +533,7 @@ void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
 		: "memory");
 }
 
-DECLARE_PER_CPU(bool, x86_ibpb_exit_to_user);
+DECLARE_PER_CPU(bool, x86_predictor_flush_exit_to_user);
 
 static inline void indirect_branch_prediction_barrier(void)
 {
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 2cb4a96247d8..002bf4adccc3 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -65,8 +65,8 @@ EXPORT_PER_CPU_SYMBOL_GPL(x86_spec_ctrl_current);
  * be needed to before running userspace. That IBPB will flush the branch
  * predictor content.
  */
-DEFINE_PER_CPU(bool, x86_ibpb_exit_to_user);
-EXPORT_PER_CPU_SYMBOL_GPL(x86_ibpb_exit_to_user);
+DEFINE_PER_CPU(bool, x86_predictor_flush_exit_to_user);
+EXPORT_PER_CPU_SYMBOL_GPL(x86_predictor_flush_exit_to_user);
 
 u64 x86_pred_cmd __ro_after_init = PRED_CMD_IBPB;
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fd1c4a36b593..45d7cfedc507 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11464,7 +11464,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	 * may migrate to.
 	 */
 	if (cpu_feature_enabled(X86_FEATURE_IBPB_EXIT_TO_USER))
-		this_cpu_write(x86_ibpb_exit_to_user, true);
+		this_cpu_write(x86_predictor_flush_exit_to_user, true);
 
 	/*
 	 * Consume any pending interrupts, including the possible source of

-- 
2.34.1

[PATCH v10 05/12] x86/vmscape: Move mitigation selection to a switch()

[Pawan Gupta]

This ensures that all mitigation modes are explicitly handled, while
keeping the mitigation selection for each mode together. This also prepares
for adding BHB-clearing mitigation mode for VMSCAPE.

Tested-by: Jon Kohler <jon@nutanix.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/kernel/cpu/bugs.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 002bf4adccc3..636280c612f0 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -3088,17 +3088,33 @@ early_param("vmscape", vmscape_parse_cmdline);
 
 static void __init vmscape_select_mitigation(void)
 {
-	if (!boot_cpu_has_bug(X86_BUG_VMSCAPE) ||
-	    !boot_cpu_has(X86_FEATURE_IBPB)) {
+	if (!boot_cpu_has_bug(X86_BUG_VMSCAPE)) {
 		vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
 		return;
 	}
 
-	if (vmscape_mitigation == VMSCAPE_MITIGATION_AUTO) {
-		if (should_mitigate_vuln(X86_BUG_VMSCAPE))
+	if ((vmscape_mitigation == VMSCAPE_MITIGATION_AUTO) &&
+	    !should_mitigate_vuln(X86_BUG_VMSCAPE))
+		vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
+
+	switch (vmscape_mitigation) {
+	case VMSCAPE_MITIGATION_NONE:
+		break;
+
+	case VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER:
+		if (!boot_cpu_has(X86_FEATURE_IBPB))
+			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
+		break;
+
+	case VMSCAPE_MITIGATION_AUTO:
+		if (boot_cpu_has(X86_FEATURE_IBPB))
 			vmscape_mitigation = VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER;
 		else
 			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
+		break;
+
+	default:
+		break;
 	}
 }
 

-- 
2.34.1

[PATCH v10 06/12] x86/vmscape: Use write_ibpb() instead of indirect_branch_prediction_barrier()

[Pawan Gupta]

indirect_branch_prediction_barrier() is a wrapper to write_ibpb(), which
also checks if the CPU supports IBPB. For VMSCAPE, call to
indirect_branch_prediction_barrier() is only possible when CPU supports
IBPB.

Simply call write_ibpb() directly to avoid unnecessary alternative
patching.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Jon Kohler <jon@nutanix.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/include/asm/entry-common.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index c45858db16c9..78b143673ca7 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -97,7 +97,7 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 	/* Avoid unnecessary reads of 'x86_predictor_flush_exit_to_user' */
 	if (cpu_feature_enabled(X86_FEATURE_IBPB_EXIT_TO_USER) &&
 	    this_cpu_read(x86_predictor_flush_exit_to_user)) {
-		indirect_branch_prediction_barrier();
+		write_ibpb();
 		this_cpu_write(x86_predictor_flush_exit_to_user, false);
 	}
 }

-- 
2.34.1

[PATCH v10 07/12] static_call: Add EXPORT_STATIC_CALL_FOR_MODULES()

[Pawan Gupta]

There is EXPORT_STATIC_CALL_TRAMP() that hides the static key from all
modules. But there is no equivalent of EXPORT_SYMBOL_FOR_MODULES() to
restrict symbol visibility to only certain modules.

Add EXPORT_STATIC_CALL_FOR_MODULES(name, mods) that wraps both the key and
the trampoline with EXPORT_SYMBOL_FOR_MODULES(), allowing only a limited
set of modules to see and update the static key.

The immediate user is KVM, in the following commit.

checkpatch reported below warnings with this change that I believe don't
apply in this case:

  include/linux/static_call.h:219: WARNING: Non-declarative macros with multiple statements should be enclosed in a do - while loop
  include/linux/static_call.h:220: WARNING: EXPORT_SYMBOL(foo); should immediately follow its function/variable

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 include/linux/static_call.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index 78a77a4ae0ea..b610afd1ed55 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -216,6 +216,9 @@ extern long __static_call_return0(void);
 #define EXPORT_STATIC_CALL_GPL(name)					\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name));			\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
+#define EXPORT_STATIC_CALL_FOR_MODULES(name, mods)			\
+	EXPORT_SYMBOL_FOR_MODULES(STATIC_CALL_KEY(name), mods);		\
+	EXPORT_SYMBOL_FOR_MODULES(STATIC_CALL_TRAMP(name), mods)
 
 /* Leave the key unexported, so modules can't change static call targets: */
 #define EXPORT_STATIC_CALL_TRAMP(name)					\
@@ -276,6 +279,9 @@ extern long __static_call_return0(void);
 #define EXPORT_STATIC_CALL_GPL(name)					\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name));			\
 	EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
+#define EXPORT_STATIC_CALL_FOR_MODULES(name, mods)			\
+	EXPORT_SYMBOL_FOR_MODULES(STATIC_CALL_KEY(name), mods);		\
+	EXPORT_SYMBOL_FOR_MODULES(STATIC_CALL_TRAMP(name), mods)
 
 /* Leave the key unexported, so modules can't change static call targets: */
 #define EXPORT_STATIC_CALL_TRAMP(name)					\
@@ -346,6 +352,8 @@ static inline int static_call_text_reserved(void *start, void *end)
 
 #define EXPORT_STATIC_CALL(name)	EXPORT_SYMBOL(STATIC_CALL_KEY(name))
 #define EXPORT_STATIC_CALL_GPL(name)	EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name))
+#define EXPORT_STATIC_CALL_FOR_MODULES(name, mods)			\
+	EXPORT_SYMBOL_FOR_MODULES(STATIC_CALL_KEY(name), mods)
 
 #endif /* CONFIG_HAVE_STATIC_CALL */
 

-- 
2.34.1

[PATCH v10 08/12] kvm: Define EXPORT_STATIC_CALL_FOR_KVM()

[Pawan Gupta]

EXPORT_SYMBOL_FOR_KVM() exists to export symbols to KVM modules. Static
calls need the same treatment when the core kernel defines a static_call
that KVM needs access to (e.g. from a VM-exit path).

Define EXPORT_STATIC_CALL_FOR_KVM() as the static_call analogue of
EXPORT_SYMBOL_FOR_KVM(). The same three-way logic applies:

  - KVM_SUB_MODULES defined: export to "kvm," plus all sub-modules
  - KVM=m, no sub-modules: export to "kvm" only
  - KVM built-in: no export needed (noop)

As with EXPORT_SYMBOL_FOR_KVM(), allow architectures to override the
definition (e.g. to suppress the export when kvm.ko itself will not be
built despite CONFIG_KVM=m). Add the x86 no-op override in
arch/x86/include/asm/kvm_types.h for that case.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/include/asm/kvm_types.h |  1 +
 include/linux/kvm_types.h        | 13 ++++++++++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_types.h b/arch/x86/include/asm/kvm_types.h
index d7c704ed1be9..bceeaed2940e 100644
--- a/arch/x86/include/asm/kvm_types.h
+++ b/arch/x86/include/asm/kvm_types.h
@@ -15,6 +15,7 @@
  * at least one vendor module is enabled.
  */
 #define EXPORT_SYMBOL_FOR_KVM(symbol)
+#define EXPORT_STATIC_CALL_FOR_KVM(symbol)
 #endif
 
 #define KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE 40
diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index a568d8e6f4e8..c81f4fdba625 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -13,6 +13,8 @@
 	EXPORT_SYMBOL_FOR_MODULES(symbol, __stringify(KVM_SUB_MODULES))
 #define EXPORT_SYMBOL_FOR_KVM(symbol) \
 	EXPORT_SYMBOL_FOR_MODULES(symbol, "kvm," __stringify(KVM_SUB_MODULES))
+#define EXPORT_STATIC_CALL_FOR_KVM(symbol) \
+	EXPORT_STATIC_CALL_FOR_MODULES(symbol, "kvm," __stringify(KVM_SUB_MODULES))
 #else
 #define EXPORT_SYMBOL_FOR_KVM_INTERNAL(symbol)
 /*
@@ -27,7 +29,16 @@
 #define EXPORT_SYMBOL_FOR_KVM(symbol)
 #endif /* IS_MODULE(CONFIG_KVM) */
 #endif /* EXPORT_SYMBOL_FOR_KVM */
-#endif
+
+#ifndef EXPORT_STATIC_CALL_FOR_KVM
+#if IS_MODULE(CONFIG_KVM)
+#define EXPORT_STATIC_CALL_FOR_KVM(symbol) EXPORT_STATIC_CALL_FOR_MODULES(symbol, "kvm")
+#else
+#define EXPORT_STATIC_CALL_FOR_KVM(symbol)
+#endif /* IS_MODULE(CONFIG_KVM) */
+#endif /* EXPORT_STATIC_CALL_FOR_KVM */
+
+#endif /* KVM_SUB_MODULES */
 
 #ifndef __ASSEMBLER__
 

-- 
2.34.1

Re: [PATCH v10 08/12] kvm: Define EXPORT_STATIC_CALL_FOR_KVM()

[Sean Christopherson]

On Tue, Apr 14, 2026, Pawan Gupta wrote:
> EXPORT_SYMBOL_FOR_KVM() exists to export symbols to KVM modules. Static
> calls need the same treatment when the core kernel defines a static_call
> that KVM needs access to (e.g. from a VM-exit path).
> 
> Define EXPORT_STATIC_CALL_FOR_KVM() as the static_call analogue of
> EXPORT_SYMBOL_FOR_KVM(). The same three-way logic applies:
> 
>   - KVM_SUB_MODULES defined: export to "kvm," plus all sub-modules
>   - KVM=m, no sub-modules: export to "kvm" only
>   - KVM built-in: no export needed (noop)
> 
> As with EXPORT_SYMBOL_FOR_KVM(), allow architectures to override the
> definition (e.g. to suppress the export when kvm.ko itself will not be
> built despite CONFIG_KVM=m). Add the x86 no-op override in
> arch/x86/include/asm/kvm_types.h for that case.
> 
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> ---
>  arch/x86/include/asm/kvm_types.h |  1 +
>  include/linux/kvm_types.h        | 13 ++++++++++++-
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_types.h b/arch/x86/include/asm/kvm_types.h
> index d7c704ed1be9..bceeaed2940e 100644
> --- a/arch/x86/include/asm/kvm_types.h
> +++ b/arch/x86/include/asm/kvm_types.h
> @@ -15,6 +15,7 @@
>   * at least one vendor module is enabled.
>   */
>  #define EXPORT_SYMBOL_FOR_KVM(symbol)
> +#define EXPORT_STATIC_CALL_FOR_KVM(symbol)
>  #endif
>  
>  #define KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE 40
> diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> index a568d8e6f4e8..c81f4fdba625 100644
> --- a/include/linux/kvm_types.h
> +++ b/include/linux/kvm_types.h
> @@ -13,6 +13,8 @@
>  	EXPORT_SYMBOL_FOR_MODULES(symbol, __stringify(KVM_SUB_MODULES))
>  #define EXPORT_SYMBOL_FOR_KVM(symbol) \
>  	EXPORT_SYMBOL_FOR_MODULES(symbol, "kvm," __stringify(KVM_SUB_MODULES))
> +#define EXPORT_STATIC_CALL_FOR_KVM(symbol) \
> +	EXPORT_STATIC_CALL_FOR_MODULES(symbol, "kvm," __stringify(KVM_SUB_MODULES))
>  #else
>  #define EXPORT_SYMBOL_FOR_KVM_INTERNAL(symbol)
>  /*
> @@ -27,7 +29,16 @@
>  #define EXPORT_SYMBOL_FOR_KVM(symbol)
>  #endif /* IS_MODULE(CONFIG_KVM) */
>  #endif /* EXPORT_SYMBOL_FOR_KVM */
> -#endif
> +
> +#ifndef EXPORT_STATIC_CALL_FOR_KVM
> +#if IS_MODULE(CONFIG_KVM)
> +#define EXPORT_STATIC_CALL_FOR_KVM(symbol) EXPORT_STATIC_CALL_FOR_MODULES(symbol, "kvm")
> +#else
> +#define EXPORT_STATIC_CALL_FOR_KVM(symbol)
> +#endif /* IS_MODULE(CONFIG_KVM) */
> +#endif /* EXPORT_STATIC_CALL_FOR_KVM */
> +
> +#endif /* KVM_SUB_MODULES */

I think I'd prefer to require EXPORT_SYMBOL_FOR_KVM and EXPORT_STATIC_CALL_FOR_KVM
to come as a pair from arch code.  I can't think of a scenario where arch code
should override one but not the other.  The end result is slightly less ugly :-)

---
 arch/x86/include/asm/kvm_types.h |  1 +
 include/linux/kvm_types.h        | 10 +++++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_types.h b/arch/x86/include/asm/kvm_types.h
index d7c704ed1be9..bceeaed2940e 100644
--- a/arch/x86/include/asm/kvm_types.h
+++ b/arch/x86/include/asm/kvm_types.h
@@ -15,6 +15,7 @@
  * at least one vendor module is enabled.
  */
 #define EXPORT_SYMBOL_FOR_KVM(symbol)
+#define EXPORT_STATIC_CALL_FOR_KVM(symbol)
 #endif
 
 #define KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE 40
diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index a568d8e6f4e8..be602d3f287e 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -13,6 +13,8 @@
 	EXPORT_SYMBOL_FOR_MODULES(symbol, __stringify(KVM_SUB_MODULES))
 #define EXPORT_SYMBOL_FOR_KVM(symbol) \
 	EXPORT_SYMBOL_FOR_MODULES(symbol, "kvm," __stringify(KVM_SUB_MODULES))
+#define EXPORT_STATIC_CALL_FOR_KVM(symbol) \
+	EXPORT_STATIC_CALL_FOR_MODULES(symbol, "kvm," __stringify(KVM_SUB_MODULES))
 #else
 #define EXPORT_SYMBOL_FOR_KVM_INTERNAL(symbol)
 /*
@@ -23,11 +25,17 @@
 #ifndef EXPORT_SYMBOL_FOR_KVM
 #if IS_MODULE(CONFIG_KVM)
 #define EXPORT_SYMBOL_FOR_KVM(symbol) EXPORT_SYMBOL_FOR_MODULES(symbol, "kvm")
+#define EXPORT_STATIC_CALL_FOR_KVM(symbol) EXPORT_STATIC_CALL_FOR_MODULES(symbol, "kvm")
 #else
 #define EXPORT_SYMBOL_FOR_KVM(symbol)
+#define EXPORT_STATIC_CALL_FOR_KVM(symbol)
 #endif /* IS_MODULE(CONFIG_KVM) */
+#else
+#ifndef EXPORT_STATIC_CALL_FOR_KVM
+#error Must #define EXPORT_STATIC_CALL_FOR_KVM if #defining EXPORT_SYMBOL_FOR_KVM
+#endif
 #endif /* EXPORT_SYMBOL_FOR_KVM */
-#endif
+#endif /* KVM_SUB_MODULES */
 
 #ifndef __ASSEMBLER__
 

base-commit: 56b7ace84970ff647b095849e80bc36c094760aa
--

Re: [PATCH v10 08/12] kvm: Define EXPORT_STATIC_CALL_FOR_KVM()

[Pawan Gupta]

On Thu, Apr 16, 2026 at 03:44:49PM -0700, Sean Christopherson wrote:
> I think I'd prefer to require EXPORT_SYMBOL_FOR_KVM and EXPORT_STATIC_CALL_FOR_KVM
> to come as a pair from arch code.  I can't think of a scenario where arch code
> should override one but not the other.

Right, will change as you suggested.

[PATCH v10 09/12] x86/vmscape: Use static_call() for predictor flush

[Pawan Gupta]

Adding more mitigation options at exit-to-userspace for VMSCAPE would
usually require a series of checks to decide which mitigation to use. In
this case, the mitigation is done by calling a function, which is decided
at boot. So, adding more feature flags and multiple checks can be avoided
by using static_call() to the mitigating function.

Replace the flag-based mitigation selector with a static_call(). This also
frees the existing X86_FEATURE_IBPB_EXIT_TO_USER.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Jon Kohler <jon@nutanix.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/Kconfig                     | 1 +
 arch/x86/include/asm/cpufeatures.h   | 2 +-
 arch/x86/include/asm/entry-common.h  | 7 +++----
 arch/x86/include/asm/nospec-branch.h | 3 +++
 arch/x86/kernel/cpu/bugs.c           | 9 ++++++++-
 arch/x86/kvm/x86.c                   | 2 +-
 6 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e2df1b147184..5b8def9ddb98 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2720,6 +2720,7 @@ config MITIGATION_TSA
 config MITIGATION_VMSCAPE
 	bool "Mitigate VMSCAPE"
 	depends on KVM
+	depends on HAVE_STATIC_CALL
 	default y
 	help
 	  Enable mitigation for VMSCAPE attacks. VMSCAPE is a hardware security
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index dbe104df339b..b4d529dd6d30 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -503,7 +503,7 @@
 #define X86_FEATURE_TSA_SQ_NO		(21*32+11) /* AMD CPU not vulnerable to TSA-SQ */
 #define X86_FEATURE_TSA_L1_NO		(21*32+12) /* AMD CPU not vulnerable to TSA-L1 */
 #define X86_FEATURE_CLEAR_CPU_BUF_VM	(21*32+13) /* Clear CPU buffers using VERW before VMRUN */
-#define X86_FEATURE_IBPB_EXIT_TO_USER	(21*32+14) /* Use IBPB on exit-to-userspace, see VMSCAPE bug */
+/* Free */
 #define X86_FEATURE_ABMC		(21*32+15) /* Assignable Bandwidth Monitoring Counters */
 #define X86_FEATURE_MSR_IMM		(21*32+16) /* MSR immediate form instructions */
 #define X86_FEATURE_SGX_EUPDATESVN	(21*32+17) /* Support for ENCLS[EUPDATESVN] instruction */
diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 78b143673ca7..783e7cb50cae 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -4,6 +4,7 @@
 
 #include <linux/randomize_kstack.h>
 #include <linux/user-return-notifier.h>
+#include <linux/static_call_types.h>
 
 #include <asm/nospec-branch.h>
 #include <asm/io_bitmap.h>
@@ -94,10 +95,8 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 	 */
 	choose_random_kstack_offset(rdtsc());
 
-	/* Avoid unnecessary reads of 'x86_predictor_flush_exit_to_user' */
-	if (cpu_feature_enabled(X86_FEATURE_IBPB_EXIT_TO_USER) &&
-	    this_cpu_read(x86_predictor_flush_exit_to_user)) {
-		write_ibpb();
+	if (unlikely(this_cpu_read(x86_predictor_flush_exit_to_user))) {
+		static_call_cond(vmscape_predictor_flush)();
 		this_cpu_write(x86_predictor_flush_exit_to_user, false);
 	}
 }
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 0381db59c39d..066fd8095200 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -542,6 +542,9 @@ static inline void indirect_branch_prediction_barrier(void)
 			    :: "rax", "rcx", "rdx", "memory");
 }
 
+#include <linux/static_call_types.h>
+DECLARE_STATIC_CALL(vmscape_predictor_flush, write_ibpb);
+
 /* The Intel SPEC CTRL MSR base value cache */
 extern u64 x86_spec_ctrl_base;
 DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 636280c612f0..bfc0e41697f6 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -144,6 +144,13 @@ EXPORT_SYMBOL_GPL(cpu_buf_idle_clear);
  */
 DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
 
+/*
+ * Controls how vmscape is mitigated e.g. via IBPB or BHB-clear
+ * sequence. This defaults to no mitigation.
+ */
+DEFINE_STATIC_CALL_NULL(vmscape_predictor_flush, write_ibpb);
+EXPORT_STATIC_CALL_FOR_KVM(vmscape_predictor_flush);
+
 #undef pr_fmt
 #define pr_fmt(fmt)	"mitigations: " fmt
 
@@ -3133,7 +3140,7 @@ static void __init vmscape_update_mitigation(void)
 static void __init vmscape_apply_mitigation(void)
 {
 	if (vmscape_mitigation == VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER)
-		setup_force_cpu_cap(X86_FEATURE_IBPB_EXIT_TO_USER);
+		static_call_update(vmscape_predictor_flush, write_ibpb);
 }
 
 #undef pr_fmt
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 45d7cfedc507..5582056b2fa1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11463,7 +11463,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	 * set for the CPU that actually ran the guest, and not the CPU that it
 	 * may migrate to.
 	 */
-	if (cpu_feature_enabled(X86_FEATURE_IBPB_EXIT_TO_USER))
+	if (static_call_query(vmscape_predictor_flush))
 		this_cpu_write(x86_predictor_flush_exit_to_user, true);
 
 	/*

-- 
2.34.1

Re: [PATCH v10 09/12] x86/vmscape: Use static_call() for predictor flush

[Sean Christopherson]

On Tue, Apr 14, 2026, Pawan Gupta wrote:
> Adding more mitigation options at exit-to-userspace for VMSCAPE would
> usually require a series of checks to decide which mitigation to use. In
> this case, the mitigation is done by calling a function, which is decided
> at boot. So, adding more feature flags and multiple checks can be avoided
> by using static_call() to the mitigating function.
> 
> Replace the flag-based mitigation selector with a static_call(). This also
> frees the existing X86_FEATURE_IBPB_EXIT_TO_USER.
> 
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Tested-by: Jon Kohler <jon@nutanix.com>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> ---

For the KVM change,

Acked-by: Sean Christopherson <seanjc@google.com>

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 45d7cfedc507..5582056b2fa1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11463,7 +11463,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	 * set for the CPU that actually ran the guest, and not the CPU that it
>  	 * may migrate to.
>  	 */
> -	if (cpu_feature_enabled(X86_FEATURE_IBPB_EXIT_TO_USER))
> +	if (static_call_query(vmscape_predictor_flush))
>  		this_cpu_write(x86_predictor_flush_exit_to_user, true);
>  
>  	/*

[PATCH v10 10/12] x86/vmscape: Deploy BHB clearing mitigation

[Pawan Gupta]

IBPB mitigation for VMSCAPE is an overkill on CPUs that are only affected
by the BHI variant of VMSCAPE. On such CPUs, eIBRS already provides
indirect branch isolation between guest and host userspace. However, branch
history from guest may also influence the indirect branches in host
userspace.

To mitigate the BHI aspect, use the BHB clearing sequence. Since now, IBPB
is not the only mitigation for VMSCAPE, update the documentation to reflect
that =auto could select either IBPB or BHB clear mitigation based on the
CPU.

Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Tested-by: Jon Kohler <jon@nutanix.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 Documentation/admin-guide/hw-vuln/vmscape.rst   | 11 ++++++++-
 Documentation/admin-guide/kernel-parameters.txt |  4 +++-
 arch/x86/include/asm/entry-common.h             |  4 ++++
 arch/x86/include/asm/nospec-branch.h            |  2 ++
 arch/x86/kernel/cpu/bugs.c                      | 30 +++++++++++++++++++------
 5 files changed, 42 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/hw-vuln/vmscape.rst b/Documentation/admin-guide/hw-vuln/vmscape.rst
index d9b9a2b6c114..7c40cf70ad7a 100644
--- a/Documentation/admin-guide/hw-vuln/vmscape.rst
+++ b/Documentation/admin-guide/hw-vuln/vmscape.rst
@@ -86,6 +86,10 @@ The possible values in this file are:
    run a potentially malicious guest and issues an IBPB before the first
    exit to userspace after VM-exit.
 
+ * 'Mitigation: Clear BHB before exit to userspace':
+
+   As above, conditional BHB clearing mitigation is enabled.
+
  * 'Mitigation: IBPB on VMEXIT':
 
    IBPB is issued on every VM-exit. This occurs when other mitigations like
@@ -102,9 +106,14 @@ The mitigation can be controlled via the ``vmscape=`` command line parameter:
 
  * ``vmscape=ibpb``:
 
-   Enable conditional IBPB mitigation (default when CONFIG_MITIGATION_VMSCAPE=y).
+   Enable conditional IBPB mitigation.
 
  * ``vmscape=force``:
 
    Force vulnerability detection and mitigation even on processors that are
    not known to be affected.
+
+ * ``vmscape=auto``:
+
+   Choose the mitigation based on the VMSCAPE variant the CPU is affected by.
+   (default when CONFIG_MITIGATION_VMSCAPE=y)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 03a550630644..3853c7109419 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -8378,9 +8378,11 @@ Kernel parameters
 
 			off		- disable the mitigation
 			ibpb		- use Indirect Branch Prediction Barrier
-					  (IBPB) mitigation (default)
+					  (IBPB) mitigation
 			force		- force vulnerability detection even on
 					  unaffected processors
+			auto		- (default) use IBPB or BHB clear
+					  mitigation based on CPU
 
 	vsyscall=	[X86-64,EARLY]
 			Controls the behavior of vsyscalls (i.e. calls to
diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 783e7cb50cae..13db31472f3a 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -96,6 +96,10 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 	choose_random_kstack_offset(rdtsc());
 
 	if (unlikely(this_cpu_read(x86_predictor_flush_exit_to_user))) {
+		/*
+		 * Since the mitigation is for userspace, an explicit
+		 * speculation barrier is not required after flush.
+		 */
 		static_call_cond(vmscape_predictor_flush)();
 		this_cpu_write(x86_predictor_flush_exit_to_user, false);
 	}
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 066fd8095200..38478383139b 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -390,6 +390,8 @@ extern void write_ibpb(void);
 
 #ifdef CONFIG_X86_64
 extern void clear_bhb_loop_nofence(void);
+#else
+static inline void clear_bhb_loop_nofence(void) {}
 #endif
 
 extern void (*x86_return_thunk)(void);
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index bfc0e41697f6..1082ed1fb2e6 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -61,9 +61,8 @@ DEFINE_PER_CPU(u64, x86_spec_ctrl_current);
 EXPORT_PER_CPU_SYMBOL_GPL(x86_spec_ctrl_current);
 
 /*
- * Set when the CPU has run a potentially malicious guest. An IBPB will
- * be needed to before running userspace. That IBPB will flush the branch
- * predictor content.
+ * Set when the CPU has run a potentially malicious guest. Indicates that a
+ * branch predictor flush is needed before running userspace.
  */
 DEFINE_PER_CPU(bool, x86_predictor_flush_exit_to_user);
 EXPORT_PER_CPU_SYMBOL_GPL(x86_predictor_flush_exit_to_user);
@@ -3061,13 +3060,15 @@ enum vmscape_mitigations {
 	VMSCAPE_MITIGATION_AUTO,
 	VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER,
 	VMSCAPE_MITIGATION_IBPB_ON_VMEXIT,
+	VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER,
 };
 
 static const char * const vmscape_strings[] = {
-	[VMSCAPE_MITIGATION_NONE]		= "Vulnerable",
+	[VMSCAPE_MITIGATION_NONE]			= "Vulnerable",
 	/* [VMSCAPE_MITIGATION_AUTO] */
-	[VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER]	= "Mitigation: IBPB before exit to userspace",
-	[VMSCAPE_MITIGATION_IBPB_ON_VMEXIT]	= "Mitigation: IBPB on VMEXIT",
+	[VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER]		= "Mitigation: IBPB before exit to userspace",
+	[VMSCAPE_MITIGATION_IBPB_ON_VMEXIT]		= "Mitigation: IBPB on VMEXIT",
+	[VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER]	= "Mitigation: Clear BHB before exit to userspace",
 };
 
 static enum vmscape_mitigations vmscape_mitigation __ro_after_init =
@@ -3085,6 +3086,8 @@ static int __init vmscape_parse_cmdline(char *str)
 	} else if (!strcmp(str, "force")) {
 		setup_force_cpu_bug(X86_BUG_VMSCAPE);
 		vmscape_mitigation = VMSCAPE_MITIGATION_AUTO;
+	} else if (!strcmp(str, "auto")) {
+		vmscape_mitigation = VMSCAPE_MITIGATION_AUTO;
 	} else {
 		pr_err("Ignoring unknown vmscape=%s option.\n", str);
 	}
@@ -3114,7 +3117,17 @@ static void __init vmscape_select_mitigation(void)
 		break;
 
 	case VMSCAPE_MITIGATION_AUTO:
-		if (boot_cpu_has(X86_FEATURE_IBPB))
+		/*
+		 * CPUs with BHI_CTRL(ADL and newer) can avoid the IBPB and use
+		 * BHB clear sequence. These CPUs are only vulnerable to the BHI
+		 * variant of the VMSCAPE attack, and thus they do not require a
+		 * full predictor flush.
+		 *
+		 * Note, in 32-bit mode BHB clear sequence is not supported.
+		 */
+		if (boot_cpu_has(X86_FEATURE_BHI_CTRL) && IS_ENABLED(CONFIG_X86_64))
+			vmscape_mitigation = VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER;
+		else if (boot_cpu_has(X86_FEATURE_IBPB))
 			vmscape_mitigation = VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER;
 		else
 			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
@@ -3141,6 +3154,8 @@ static void __init vmscape_apply_mitigation(void)
 {
 	if (vmscape_mitigation == VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER)
 		static_call_update(vmscape_predictor_flush, write_ibpb);
+	else if (vmscape_mitigation == VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER)
+		static_call_update(vmscape_predictor_flush, clear_bhb_loop_nofence);
 }
 
 #undef pr_fmt
@@ -3232,6 +3247,7 @@ void cpu_bugs_smt_update(void)
 		break;
 	case VMSCAPE_MITIGATION_IBPB_ON_VMEXIT:
 	case VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER:
+	case VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER:
 		/*
 		 * Hypervisors can be attacked across-threads, warn for SMT when
 		 * STIBP is not already enabled system-wide.

-- 
2.34.1

[PATCH v10 11/12] x86/vmscape: Resolve conflict between attack-vectors and vmscape=force

[Pawan Gupta]

vmscape=force option currently defaults to AUTO mitigation. This lets
attack-vector controls to override the vmscape mitigation. Preventing the
user from being able to force VMSCAPE mitigation.

When vmscape mitigation is forced, allow it be deployed irrespective of
attack vectors. Introduce VMSCAPE_MITIGATION_ON that wins over
attack-vector controls.

Tested-by: Jon Kohler <jon@nutanix.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 arch/x86/kernel/cpu/bugs.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 1082ed1fb2e6..fbdb137720c4 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -3058,6 +3058,7 @@ static void __init srso_apply_mitigation(void)
 enum vmscape_mitigations {
 	VMSCAPE_MITIGATION_NONE,
 	VMSCAPE_MITIGATION_AUTO,
+	VMSCAPE_MITIGATION_ON,
 	VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER,
 	VMSCAPE_MITIGATION_IBPB_ON_VMEXIT,
 	VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER,
@@ -3066,6 +3067,7 @@ enum vmscape_mitigations {
 static const char * const vmscape_strings[] = {
 	[VMSCAPE_MITIGATION_NONE]			= "Vulnerable",
 	/* [VMSCAPE_MITIGATION_AUTO] */
+	/* [VMSCAPE_MITIGATION_ON] */
 	[VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER]		= "Mitigation: IBPB before exit to userspace",
 	[VMSCAPE_MITIGATION_IBPB_ON_VMEXIT]		= "Mitigation: IBPB on VMEXIT",
 	[VMSCAPE_MITIGATION_BHB_CLEAR_EXIT_TO_USER]	= "Mitigation: Clear BHB before exit to userspace",
@@ -3085,7 +3087,7 @@ static int __init vmscape_parse_cmdline(char *str)
 		vmscape_mitigation = VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER;
 	} else if (!strcmp(str, "force")) {
 		setup_force_cpu_bug(X86_BUG_VMSCAPE);
-		vmscape_mitigation = VMSCAPE_MITIGATION_AUTO;
+		vmscape_mitigation = VMSCAPE_MITIGATION_ON;
 	} else if (!strcmp(str, "auto")) {
 		vmscape_mitigation = VMSCAPE_MITIGATION_AUTO;
 	} else {
@@ -3117,6 +3119,7 @@ static void __init vmscape_select_mitigation(void)
 		break;
 
 	case VMSCAPE_MITIGATION_AUTO:
+	case VMSCAPE_MITIGATION_ON:
 		/*
 		 * CPUs with BHI_CTRL(ADL and newer) can avoid the IBPB and use
 		 * BHB clear sequence. These CPUs are only vulnerable to the BHI
@@ -3244,6 +3247,7 @@ void cpu_bugs_smt_update(void)
 	switch (vmscape_mitigation) {
 	case VMSCAPE_MITIGATION_NONE:
 	case VMSCAPE_MITIGATION_AUTO:
+	case VMSCAPE_MITIGATION_ON:
 		break;
 	case VMSCAPE_MITIGATION_IBPB_ON_VMEXIT:
 	case VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER:

-- 
2.34.1

[PATCH v10 12/12] x86/vmscape: Add cmdline vmscape=on to override attack vector controls

[Pawan Gupta]

In general, individual mitigation knobs override the attack vector
controls. For VMSCAPE, =ibpb exists but nothing to select BHB clearing
mitigation. The =force option would select BHB clearing when supported, but
with a side-effect of also forcing the bug, hence deploying the mitigation
on unaffected parts too.

Add a new cmdline option vmscape=on to enable the mitigation based on the
VMSCAPE variant the CPU is affected by.

Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Tested-by: Jon Kohler <jon@nutanix.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
 Documentation/admin-guide/hw-vuln/vmscape.rst   | 4 ++++
 Documentation/admin-guide/kernel-parameters.txt | 2 ++
 arch/x86/kernel/cpu/bugs.c                      | 2 ++
 3 files changed, 8 insertions(+)

diff --git a/Documentation/admin-guide/hw-vuln/vmscape.rst b/Documentation/admin-guide/hw-vuln/vmscape.rst
index 7c40cf70ad7a..2558a5c3d956 100644
--- a/Documentation/admin-guide/hw-vuln/vmscape.rst
+++ b/Documentation/admin-guide/hw-vuln/vmscape.rst
@@ -117,3 +117,7 @@ The mitigation can be controlled via the ``vmscape=`` command line parameter:
 
    Choose the mitigation based on the VMSCAPE variant the CPU is affected by.
    (default when CONFIG_MITIGATION_VMSCAPE=y)
+
+ * ``vmscape=on``:
+
+   Same as ``auto``, except that it overrides attack vector controls.
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 3853c7109419..98204d464477 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -8383,6 +8383,8 @@ Kernel parameters
 					  unaffected processors
 			auto		- (default) use IBPB or BHB clear
 					  mitigation based on CPU
+			on		- same as "auto", but override attack
+					  vector control
 
 	vsyscall=	[X86-64,EARLY]
 			Controls the behavior of vsyscalls (i.e. calls to
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index fbdb137720c4..4e0b77fb21dd 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -3088,6 +3088,8 @@ static int __init vmscape_parse_cmdline(char *str)
 	} else if (!strcmp(str, "force")) {
 		setup_force_cpu_bug(X86_BUG_VMSCAPE);
 		vmscape_mitigation = VMSCAPE_MITIGATION_ON;
+	} else if (!strcmp(str, "on")) {
+		vmscape_mitigation = VMSCAPE_MITIGATION_ON;
 	} else if (!strcmp(str, "auto")) {
 		vmscape_mitigation = VMSCAPE_MITIGATION_AUTO;
 	} else {

-- 
2.34.1