KVM: arm64: pvtime: Fix stolen time accounting across migration

When updating the stolen time we should always read the current
stolen time from the user provided memory, not from a kernel
cache. If we use a cache then we'll end up resetting stolen time
to zero on the first update after migration.
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200804170604.42662-5-drjones@redhat.com

arch/arm64/include/asm/kvm_host.h

@@ -368,7 +368,6 @@ struct kvm_vcpu_arch {
 
 	/* Guest PV state */
 	struct {
-		u64 steal;
 		u64 last_steal;
 		gpa_t base;
 	} steal;

arch/arm64/kvm/pvtime.c

@@ -13,26 +13,22 @@
 void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
 {
 	struct kvm *kvm = vcpu->kvm;
+	u64 base = vcpu->arch.steal.base;
 	u64 last_steal = vcpu->arch.steal.last_steal;
-	u64 steal;
-	__le64 steal_le;
-	u64 offset;
+	u64 offset = offsetof(struct pvclock_vcpu_stolen_time, stolen_time);
+	u64 steal = 0;
 	int idx;
-	u64 base = vcpu->arch.steal.base;
 
 	if (base == GPA_INVALID)
 		return;
 
-	/* Let's do the local bookkeeping */
-	steal = vcpu->arch.steal.steal;
-	vcpu->arch.steal.last_steal = READ_ONCE(current->sched_info.run_delay);
-	steal += vcpu->arch.steal.last_steal - last_steal;
-	vcpu->arch.steal.steal = steal;
-
-	steal_le = cpu_to_le64(steal);
 	idx = srcu_read_lock(&kvm->srcu);
-	offset = offsetof(struct pvclock_vcpu_stolen_time, stolen_time);
-	kvm_put_guest(kvm, base + offset, steal_le);
+	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);
+		steal += vcpu->arch.steal.last_steal - last_steal;
+		kvm_put_guest(kvm, base + offset, cpu_to_le64(steal));
+	}
 	srcu_read_unlock(&kvm->srcu, idx);
 }
 
@@ -66,7 +62,6 @@ gpa_t kvm_init_stolen_time(struct kvm_vcpu *vcpu)
 	 * Start counting stolen time from the time the guest requests
 	 * the feature enabled.
 	 */
-	vcpu->arch.steal.steal = 0;
 	vcpu->arch.steal.last_steal = current->sched_info.run_delay;
 
 	idx = srcu_read_lock(&kvm->srcu);

include/linux/kvm_host.h

@@ -749,6 +749,26 @@ int kvm_write_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 			      gpa_t gpa, unsigned long len);
 
+#define __kvm_get_guest(kvm, gfn, offset, v)				\
+({									\
+	unsigned long __addr = gfn_to_hva(kvm, gfn);			\
+	typeof(v) __user *__uaddr = (typeof(__uaddr))(__addr + offset);	\
+	int __ret = -EFAULT;						\
+									\
+	if (!kvm_is_error_hva(__addr))					\
+		__ret = get_user(v, __uaddr);				\
+	__ret;								\
+})
+
+#define kvm_get_guest(kvm, gpa, v)					\
+({									\
+	gpa_t __gpa = gpa;						\
+	struct kvm *__kvm = kvm;					\
+									\
+	__kvm_get_guest(__kvm, __gpa >> PAGE_SHIFT,			\
+			offset_in_page(__gpa), v);			\
+})
+
 #define __kvm_put_guest(kvm, gfn, offset, v)				\
 ({									\
 	unsigned long __addr = gfn_to_hva(kvm, gfn);			\