linux-kernel - Re: [PATCH v2] KVM: nVMX: Skip IBPB when switching between vmcs01 and vmcs02

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Message-ID: <9d0d09da-3920-16d6-11ae-51b864171b66@redhat.com>
Date:   Mon, 4 May 2020 15:12:36 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Alexander Graf <graf@...zon.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        KarimAllah Raslan <karahmed@...zon.de>
Subject: Re: [PATCH v2] KVM: nVMX: Skip IBPB when switching between vmcs01 and
 vmcs02

On 04/05/20 14:01, Alexander Graf wrote:
> I like the WARN_ON :). It should be almost free during execution, but
> helps us catch problems early.

Yes, it's nice.  I didn't mind the "buddy" argument either, but if we're 
going to get a bool I prefer positive logic so I'd like to squash this:

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index b57420f3dd8f..299393750a18 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -304,7 +304,13 @@ static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct loaded_vmcs *vmcs)
 	prev = vmx->loaded_vmcs;
 	WARN_ON_ONCE(prev->cpu != cpu || prev->vmcs != per_cpu(current_vmcs, cpu));
 	vmx->loaded_vmcs = vmcs;
-	vmx_vcpu_load_vmcs(vcpu, cpu, true);
+
+	/*
+	 * This is the same guest from our point of view, so no
+	 * indirect branch prediction barrier is needed.  The L1
+	 * guest can protect itself with retpolines, IBPB or IBRS.
+	 */
+	vmx_vcpu_load_vmcs(vcpu, cpu, false);
 	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 669e14947ba9..0f9c8d2dd7f6 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1311,7 +1311,7 @@ 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, bool nested_switch)
+void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, bool need_ibpb)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 	bool already_loaded = vmx->loaded_vmcs->cpu == cpu;
@@ -1336,7 +1336,7 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, bool nested_switch)
 	if (per_cpu(current_vmcs, cpu) != vmx->loaded_vmcs->vmcs) {
 		per_cpu(current_vmcs, cpu) = vmx->loaded_vmcs->vmcs;
 		vmcs_load(vmx->loaded_vmcs->vmcs);
-		if (!nested_switch)
+		if (need_ibpb)
 			indirect_branch_prediction_barrier();
 	}
 
@@ -1378,7 +1378,7 @@ void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	vmx_vcpu_load_vmcs(vcpu, cpu, false);
+	vmx_vcpu_load_vmcs(vcpu, cpu, true);
 
 	vmx_vcpu_pi_load(vcpu, cpu);
 
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index fa61dc802183..e584ee9b3e94 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -320,7 +320,7 @@ struct kvm_vmx {
 };
 
 bool nested_vmx_allowed(struct kvm_vcpu *vcpu);
-void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, bool nested_switch);
+void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu, bool need_ibpb);
 void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu);
 int allocate_vpid(void);
 void free_vpid(int vpid);


Paolo