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x86/bugs: Use a static branch to guard IBPB on vCPU switch

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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>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Acked-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20250227012712.3193063-5-yosry.ahmed@linux.dev
This commit is contained in:
Yosry Ahmed 2025-02-27 01:27:10 +00:00 committed by Ingo Molnar
parent bd9a8542ce
commit 80dacb0804
4 changed files with 9 additions and 2 deletions
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2
arch/x86/include/asm/nospec-branch.h
View File

@ -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);

5
arch/x86/kernel/cpu/bugs.c
View File

@ -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) {

2
arch/x86/kvm/svm/svm.c
View File

@ -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))

2
arch/x86/kvm/vmx/vmx.c
View File

@ -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();
}