The patch below does not apply to the 4.14-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 5c911beff20aa8639e7a1f28988736c13e03ed54 Mon Sep 17 00:00:00 2001

From: Sean Christopherson sean.j.christopherson@intel.com Date: Fri, 1 May 2020 09:31:17 -0700 Subject: [PATCH] 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 669445136144..7f754b3bc6dd 100644 --- 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 fbede089d0f7..db842ce74e5d 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -1311,10 +1311,12 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu) 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 b5e773267abe..d3d48acc6bd9 100644 --- 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);