Commit 5c911bef authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

KVM: nVMX: Skip IBPB when switching between vmcs01 and vmcs02 



Skip the Indirect Branch Prediction Barrier that is triggered on a VMCS
switch when running with spectre_v2_user=on/auto if the switch is
between two VMCSes in the same guest, i.e. between vmcs01 and vmcs02.
The IBPB is intended to prevent one guest from attacking another, which
is unnecessary in the nested case as it's the same guest from KVM's
perspective.

This all but eliminates the overhead observed for nested VMX transitions
when running with CONFIG_RETPOLINE=y and spectre_v2_user=on/auto, which
can be significant, e.g. roughly 3x on current systems.

Reported-by: default avatarAlexander Graf <graf@amazon.com>
Cc: KarimAllah Raslan <karahmed@amazon.de>
Cc: stable@vger.kernel.org
Fixes: 15d45071 ("KVM/x86: Add IBPB support")
Signed-off-by: default avatarSean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200501163117.4655-1-sean.j.christopherson@intel.com>
[Invert direction of bool argument. - Paolo]
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent f27ad73a

arch/x86/kvm/vmx/nested.c

+1 −1
Original line number Diff line number Diff line
@@ -303,7 +303,7 @@ static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct loaded_vmcs *vmcs) 
	cpu = get_cpu();

	prev = vmx->loaded_vmcs;

	vmx->loaded_vmcs = vmcs;

	vmx_vcpu_load_vmcs(vcpu, cpu);

	vmx_vcpu_load_vmcs(vcpu, cpu, prev);

	vmx_sync_vmcs_host_state(vmx, prev);

	put_cpu();

arch/x86/kvm/vmx/vmx.c

+14 −4
Original line number Diff line number Diff line
@@ -1311,10 +1311,12 @@ after_clear_sn: 
		pi_set_on(pi_desc);

}



void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu)

void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,

			struct loaded_vmcs *buddy)

{

	struct vcpu_vmx *vmx = to_vmx(vcpu);

	bool already_loaded = vmx->loaded_vmcs->cpu == cpu;

	struct vmcs *prev;



	if (!already_loaded) {

		loaded_vmcs_clear(vmx->loaded_vmcs);
@@ -1333,9 +1335,17 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu) 
		local_irq_enable();

	}



	if (per_cpu(current_vmcs, cpu) != vmx->loaded_vmcs->vmcs) {

	prev = per_cpu(current_vmcs, cpu);

	if (prev != vmx->loaded_vmcs->vmcs) {

		per_cpu(current_vmcs, cpu) = vmx->loaded_vmcs->vmcs;

		vmcs_load(vmx->loaded_vmcs->vmcs);



		/*

		 * No indirect branch prediction barrier needed when switching

		 * the active VMCS within a guest, e.g. on nested VM-Enter.

		 * The L1 VMM can protect itself with retpolines, IBPB or IBRS.

		 */

		if (!buddy || WARN_ON_ONCE(buddy->vmcs != prev))

			indirect_branch_prediction_barrier();

	}
@@ -1377,7 +1387,7 @@ void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu) 
{

	struct vcpu_vmx *vmx = to_vmx(vcpu);



	vmx_vcpu_load_vmcs(vcpu, cpu);

	vmx_vcpu_load_vmcs(vcpu, cpu, NULL);



	vmx_vcpu_pi_load(vcpu, cpu);

arch/x86/kvm/vmx/vmx.h

+2 −1
Original line number Diff line number Diff line
@@ -320,7 +320,8 @@ struct kvm_vmx { 
};



bool nested_vmx_allowed(struct kvm_vcpu *vcpu);

void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu);

void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,

			struct loaded_vmcs *buddy);

void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu);

int allocate_vpid(void);

void free_vpid(int vpid);

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