KVM: SVM: Move GHCB unmapping to fix RCU warning

When an SEV-ES guest is running, the GHCB is unmapped as part of the
vCPU run support. However, kvm_vcpu_unmap() triggers an RCU dereference
warning with CONFIG_PROVE_LOCKING=y because the SRCU lock is released
before invoking the vCPU run support.

Move the GHCB unmapping into the prepare_guest_switch callback, which is
invoked while still holding the SRCU lock, eliminating the RCU dereference
warning.

Fixes: 291bd20 ("KVM: SVM: Add initial support for a VMGEXIT VMEXIT")
Reported-by: Borislav Petkov <[email protected]>
Signed-off-by: Tom Lendacky <[email protected]>
Message-Id: <b2f9b79d15166f2c3e4375c0d9bc3268b7696455.1620332081.git.thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: tlendacky

arch/x86/kvm/svm/sev.c

@@ -2197,7 +2197,7 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
 	return -EINVAL;
 }
 
-static void pre_sev_es_run(struct vcpu_svm *svm)
+void sev_es_unmap_ghcb(struct vcpu_svm *svm)
 {
 	if (!svm->ghcb)
 		return;
@@ -2233,9 +2233,6 @@ void pre_sev_run(struct vcpu_svm *svm, int cpu)
 	struct svm_cpu_data *sd = per_cpu(svm_data, cpu);
 	int asid = sev_get_asid(svm->vcpu.kvm);
 
-	/* Perform any SEV-ES pre-run actions */
-	pre_sev_es_run(svm);
-
 	/* Assign the asid allocated with this SEV guest */
 	svm->asid = asid;
 

arch/x86/kvm/svm/svm.c

@@ -1437,6 +1437,9 @@ static void svm_prepare_guest_switch(struct kvm_vcpu *vcpu)
 	struct vcpu_svm *svm = to_svm(vcpu);
 	struct svm_cpu_data *sd = per_cpu(svm_data, vcpu->cpu);
 
+	if (sev_es_guest(vcpu->kvm))
+		sev_es_unmap_ghcb(svm);
+
 	if (svm->guest_state_loaded)
 		return;
 

arch/x86/kvm/svm/svm.h

@@ -581,6 +581,7 @@ void sev_es_init_vmcb(struct vcpu_svm *svm);
 void sev_es_create_vcpu(struct vcpu_svm *svm);
 void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector);
 void sev_es_prepare_guest_switch(struct vcpu_svm *svm, unsigned int cpu);
+void sev_es_unmap_ghcb(struct vcpu_svm *svm);
 
 /* vmenter.S */