Commit 80dacb08 authored by Yosry Ahmed's avatar Yosry Ahmed Committed by Ingo Molnar
Browse files

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



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: default avatarYosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Acked-by: default avatarJosh Poimboeuf <jpoimboe@kernel.org>
Acked-by: default avatarSean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20250227012712.3193063-5-yosry.ahmed@linux.dev
parent bd9a8542

arch/x86/include/asm/nospec-branch.h

+2 −0
Original line number Diff line number Diff line
@@ -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);

arch/x86/kernel/cpu/bugs.c

+5 −0
Original line number Diff line number Diff line
@@ -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) {

arch/x86/kvm/svm/svm.c

+1 −1
Original line number Diff line number Diff line
@@ -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))

arch/x86/kvm/vmx/vmx.c

+1 −1
Original line number Diff line number Diff line
@@ -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();

	}