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: Alexander Graf <graf@amazon.com>
Cc: KarimAllah Raslan <karahmed@amazon.de>
Cc: stable@vger.kernel.org
Fixes: 15d45071523d ("KVM/x86: Add IBPB support")
Signed-off-by: Sean 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: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 669445136144d27ec0aea29bbbc17e1e7024c8b4..7f754b3bc6dd6266d73985c0db8a9c249fc13495 100644 (file)
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -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();
 

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index fbede089d0f7c37e4028b888b2486908a175ce21..db842ce74e5d83d6bfced715d894de76f416228e 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -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);
@@ -1333,10 +1335,18 @@ 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);
-               indirect_branch_prediction_barrier();
+
+               /*
+                * 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();
        }
 
        if (!already_loaded) {
@@ -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);
 

diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index b5e773267abe9841cebffcf03a3c5555eb41e897..d3d48acc6bd99336acc62e878432207b7db5710d 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -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);