[PATCH v2 4/6] x86/bugs: Use a static branch to guard IBPB on vCPU switch

Yosry Ahmed
2025-02-27T01:27:42Z

Instead of using X86_FEATURE_USE_IBPB to guard the IBPB execution in KVM
when a new vCPU is loaded, introduce a static branch, similar to
switch_mm_*_ibpb.

This makes it obvious in spectre_v2_user_select_mitigation() what
exactly is being toggled, instead of the unclear X86_FEATURE_USE_IBPB
(which will be shortly removed). It also provides more fine-grained
control, making it simpler to change/add paths that control the IBPB in
the vCPU switch path without affecting other IBPBs.

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
arch/x86/include/asm/nospec-branch.h | 2 ++
arch/x86/kernel/cpu/bugs.c | 5 +++++
arch/x86/kvm/svm/svm.c | 2 +-
arch/x86/kvm/vmx/vmx.c | 2 +-
4 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 7cbb76a2434b9..36bc395bdef32 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -552,6 +552,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);

+DECLARE_STATIC_KEY_FALSE(switch_vcpu_ibpb);
+
DECLARE_STATIC_KEY_FALSE(mds_idle_clear);

DECLARE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 1d7afc40f2272..7f904d0b0b04f 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -113,6 +113,10 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
/* Control unconditional IBPB in switch_mm() */
DEFINE_STATIC_KEY_FALSE(switch_mm_always_ibpb);

+/* Control IBPB on vCPU load */
+DEFINE_STATIC_KEY_FALSE(switch_vcpu_ibpb);
+EXPORT_SYMBOL_GPL(switch_vcpu_ibpb);
+
/* Control MDS CPU buffer clear before idling (halt, mwait) */
DEFINE_STATIC_KEY_FALSE(mds_idle_clear);
EXPORT_SYMBOL_GPL(mds_idle_clear);
@@ -1365,6 +1369,7 @@ spectre_v2_user_select_mitigation(void)
/* Initialize Indirect Branch Prediction Barrier */
if (boot_cpu_has(X86_FEATURE_IBPB)) {
setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
+ static_branch_enable(&switch_vcpu_ibpb);

spectre_v2_user_ibpb = mode;
switch (cmd) {
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 57222c3b56592..a73875ffbc3df 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1566,7 +1566,7 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
sd->current_vmcb = svm->vmcb;

if (!cpu_feature_enabled(X86_FEATURE_IBPB_ON_VMEXIT) &&
- cpu_feature_enabled(X86_FEATURE_USE_IBPB))
+ static_branch_likely(&switch_vcpu_ibpb))
indirect_branch_prediction_barrier();
}
if (kvm_vcpu_apicv_active(vcpu))
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 042b7a88157b0..956f9143a3c4c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1477,7 +1477,7 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,
* performs IBPB on nested VM-Exit (a single nested transition
* may switch the active VMCS multiple times).
*/
- if (cpu_feature_enabled(X86_FEATURE_USE_IBPB) &&
+ if (static_branch_likely(&switch_vcpu_ibpb) &&
(!buddy || WARN_ON_ONCE(buddy->vmcs != prev)))
indirect_branch_prediction_barrier();
}
--
2.48.1.658.g4767266eb4-goog