[RFC PATCH 12/18] KVM: arm64: Add userfault support for steal-time

[James Houghton]

As part of KVM_RUN, we may need to write steal-time information to guest
memory. In the case that the gfn we are writing to is userfault-enabled,
we should return to userspace with fault information.

With asynchronous userfaults, this change is not necessary and merely
acts as an optimization.

Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
---
 arch/arm64/include/asm/kvm_host.h |  2 +-
 arch/arm64/kvm/arm.c              |  8 ++++++--
 arch/arm64/kvm/pvtime.c           | 11 +++++++++--
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 36b8e97bf49e..4c7bd72ba9e8 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -1166,7 +1166,7 @@ static inline bool kvm_arch_pmi_in_guest(struct kvm_vcpu *vcpu)
 
 long kvm_hypercall_pv_features(struct kvm_vcpu *vcpu);
 gpa_t kvm_init_stolen_time(struct kvm_vcpu *vcpu);
-void kvm_update_stolen_time(struct kvm_vcpu *vcpu);
+int kvm_update_stolen_time(struct kvm_vcpu *vcpu);
 
 bool kvm_arm_pvtime_supported(void);
 int kvm_arm_pvtime_set_attr(struct kvm_vcpu *vcpu,
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 59716789fe0f..4c7994e44217 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -974,8 +974,12 @@ static int check_vcpu_requests(struct kvm_vcpu *vcpu)
 		 */
 		kvm_check_request(KVM_REQ_IRQ_PENDING, vcpu);
 
-		if (kvm_check_request(KVM_REQ_RECORD_STEAL, vcpu))
-			kvm_update_stolen_time(vcpu);
+		if (kvm_check_request(KVM_REQ_RECORD_STEAL, vcpu)) {
+			int ret = kvm_update_stolen_time(vcpu);
+
+			if (ret <= 0)
+				return ret;
+		}
 
 		if (kvm_check_request(KVM_REQ_RELOAD_GICv4, vcpu)) {
 			/* The distributor enable bits were changed */
diff --git a/arch/arm64/kvm/pvtime.c b/arch/arm64/kvm/pvtime.c
index 4ceabaa4c30b..ba0164726310 100644
--- a/arch/arm64/kvm/pvtime.c
+++ b/arch/arm64/kvm/pvtime.c
@@ -10,7 +10,7 @@
 
 #include <kvm/arm_hypercalls.h>
 
-void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
+int kvm_update_stolen_time(struct kvm_vcpu *vcpu)
 {
 	struct kvm *kvm = vcpu->kvm;
 	u64 base = vcpu->arch.steal.base;
@@ -20,9 +20,14 @@ void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
 	int idx;
 
 	if (base == INVALID_GPA)
-		return;
+		return 1;
 
 	idx = srcu_read_lock(&kvm->srcu);
+	if (gfn_to_hva(kvm, base + offset) == KVM_HVA_ERR_USERFAULT) {
+		kvm_prepare_memory_fault_exit(vcpu, base + offset, PAGE_SIZE,
+					      true, false, false, true);
+		return -EFAULT;
+	}
 	if (!kvm_get_guest(kvm, base + offset, steal)) {
 		steal = le64_to_cpu(steal);
 		vcpu->arch.steal.last_steal = READ_ONCE(current->sched_info.run_delay);
@@ -30,6 +35,8 @@ void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
 		kvm_put_guest(kvm, base + offset, cpu_to_le64(steal));
 	}
 	srcu_read_unlock(&kvm->srcu, idx);
+
+	return 1;
 }
 
 long kvm_hypercall_pv_features(struct kvm_vcpu *vcpu)
-- 
2.45.2.993.g49e7a77208-goog