diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 0d5d4419139a..f106db1af5ee 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -588,6 +588,8 @@ struct kvm_s390_mem_op {
 		struct {
 			__u8 ar;	/* the access register number */
 			__u8 key;	/* access key, ignored if flag unset */
+			__u8 pad1[6];	/* ignored */
+			__u64 old_addr;	/* ignored if flag unset */
 		};
 		__u32 sida_offset; /* offset into the sida */
 		__u8 reserved[32]; /* ignored */
@@ -604,6 +606,11 @@ struct kvm_s390_mem_op {
 #define KVM_S390_MEMOP_F_CHECK_ONLY		(1ULL << 0)
 #define KVM_S390_MEMOP_F_INJECT_EXCEPTION	(1ULL << 1)
 #define KVM_S390_MEMOP_F_SKEY_PROTECTION	(1ULL << 2)
+#define KVM_S390_MEMOP_F_CMPXCHG		(1ULL << 3)
+/* flags specifying extension support */
+#define KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG 0x2
+/* Non program exception return codes (pgm codes are 16 bit) */
+#define KVM_S390_MEMOP_R_NO_XCHG		(1 << 16)
 
 /* for KVM_INTERRUPT */
 struct kvm_interrupt {
diff --git a/arch/s390/kvm/gaccess.h b/arch/s390/kvm/gaccess.h
index 9408d6cc8e2c..92a3b9fb31ec 100644
--- a/arch/s390/kvm/gaccess.h
+++ b/arch/s390/kvm/gaccess.h
@@ -206,6 +206,9 @@ int access_guest_with_key(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar,
 int access_guest_real(struct kvm_vcpu *vcpu, unsigned long gra,
 		      void *data, unsigned long len, enum gacc_mode mode);
 
+int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len,
+			       __uint128_t *old, __uint128_t new, u8 access_key);
+
 /**
  * write_guest_with_key - copy data from kernel space to guest space
  * @vcpu: virtual cpu
diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
index 0243b6e38d36..6165e761a637 100644
--- a/arch/s390/kvm/gaccess.c
+++ b/arch/s390/kvm/gaccess.c
@@ -1161,6 +1161,108 @@ int access_guest_real(struct kvm_vcpu *vcpu, unsigned long gra,
 	return rc;
 }
 
+/**
+ * cmpxchg_guest_abs_with_key() - Perform cmpxchg on guest absolute address.
+ * @kvm: Virtual machine instance.
+ * @gpa: Absolute guest address of the location to be changed.
+ * @len: Operand length of the cmpxchg, required: 1 <= len <= 16. Providing a
+ *       non power of two will result in failure.
+ * @old_addr: Pointer to old value. If the location at @gpa contains this value, the
+ *         exchange will succeed. After calling cmpxchg_guest_abs_with_key() *@old
+ *         contains the value at @gpa before the attempt to exchange the value.
+ * @new: The value to place at @gpa.
+ * @access_key: The access key to use for the guest access.
+ *
+ * Atomically exchange the value at @gpa by @new, if it contains *@old.
+ * Honors storage keys.
+ *
+ * Return: * 0: successful exchange
+ *         * 1: exchange unsuccessful
+ *         * a program interruption code indicating the reason cmpxchg could
+ *           not be attempted
+ *         * -EINVAL: address misaligned or len not power of two
+ *         * -EAGAIN: transient failure (len 1 or 2)
+ *         * -EOPNOTSUPP: read-only memslot (should never occur)
+ */
+int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len,
+			       __uint128_t *old_addr, __uint128_t new,
+			       u8 access_key)
+{
+	gfn_t gfn = gpa >> PAGE_SHIFT;
+	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+	bool writable;
+	hva_t hva;
+	int ret;
+
+	if (!IS_ALIGNED(gpa, len))
+		return -EINVAL;
+
+	hva = gfn_to_hva_memslot_prot(slot, gfn, &writable);
+	if (kvm_is_error_hva(hva))
+		return PGM_ADDRESSING;
+	/*
+	 * Check if it's a read-only memslot, even though that cannot occur
+	 * since those are unsupported.
+	 * Don't try to actually handle that case.
+	 */
+	if (!writable)
+		return -EOPNOTSUPP;
+
+	hva += offset_in_page(gpa);
+	switch (len) {
+	case 1: {
+		u8 old;
+
+		ret = cmpxchg_user_key((u8 *)hva, &old, *old_addr, new, access_key);
+		ret = ret < 0 ? ret : old != *old_addr;
+		*old_addr = old;
+		break;
+	}
+	case 2: {
+		u16 old;
+
+		ret = cmpxchg_user_key((u16 *)hva, &old, *old_addr, new, access_key);
+		ret = ret < 0 ? ret : old != *old_addr;
+		*old_addr = old;
+		break;
+	}
+	case 4: {
+		u32 old;
+
+		ret = cmpxchg_user_key((u32 *)hva, &old, *old_addr, new, access_key);
+		ret = ret < 0 ? ret : old != *old_addr;
+		*old_addr = old;
+		break;
+	}
+	case 8: {
+		u64 old;
+
+		ret = cmpxchg_user_key((u64 *)hva, &old, *old_addr, new, access_key);
+		ret = ret < 0 ? ret : old != *old_addr;
+		*old_addr = old;
+		break;
+	}
+	case 16: {
+		__uint128_t old;
+
+		ret = cmpxchg_user_key((__uint128_t *)hva, &old, *old_addr, new, access_key);
+		ret = ret < 0 ? ret : old != *old_addr;
+		*old_addr = old;
+		break;
+	}
+	default:
+		return -EINVAL;
+	}
+	mark_page_dirty_in_slot(kvm, slot, gfn);
+	/*
+	 * Assume that the fault is caused by protection, either key protection
+	 * or user page write protection.
+	 */
+	if (ret == -EFAULT)
+		ret = PGM_PROTECTION;
+	return ret;
+}
+
 /**
  * guest_translate_address_with_key - translate guest logical into guest absolute address
  * @vcpu: virtual cpu
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 45d4b8182b07..47bcf2cb4345 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -576,7 +576,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_S390_VCPU_RESETS:
 	case KVM_CAP_SET_GUEST_DEBUG:
 	case KVM_CAP_S390_DIAG318:
-	case KVM_CAP_S390_MEM_OP_EXTENSION:
 		r = 1;
 		break;
 	case KVM_CAP_SET_GUEST_DEBUG2:
@@ -590,6 +589,14 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_S390_MEM_OP:
 		r = MEM_OP_MAX_SIZE;
 		break;
+	case KVM_CAP_S390_MEM_OP_EXTENSION:
+		/*
+		 * Flag bits indicating which extensions are supported.
+		 * The first extension doesn't use a flag, but pretend it does,
+		 * this way that can be changed in the future.
+		 */
+		r = KVM_S390_MEMOP_EXTENSION_CAP_CMPXCHG | 1;
+		break;
 	case KVM_CAP_NR_VCPUS:
 	case KVM_CAP_MAX_VCPUS:
 	case KVM_CAP_MAX_VCPU_ID:
@@ -2714,12 +2721,19 @@ static bool access_key_invalid(u8 access_key)
 static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
 {
 	void __user *uaddr = (void __user *)mop->buf;
+	void __user *old_addr = (void __user *)mop->old_addr;
+	union {
+		__uint128_t quad;
+		char raw[sizeof(__uint128_t)];
+	} old = { .quad = 0}, new = { .quad = 0 };
+	unsigned int off_in_quad = sizeof(new) - mop->size;
 	u64 supported_flags;
 	void *tmpbuf = NULL;
 	int r, srcu_idx;
 
 	supported_flags = KVM_S390_MEMOP_F_SKEY_PROTECTION
-			  | KVM_S390_MEMOP_F_CHECK_ONLY;
+			  | KVM_S390_MEMOP_F_CHECK_ONLY
+			  | KVM_S390_MEMOP_F_CMPXCHG;
 	if (mop->flags & ~supported_flags || !mop->size)
 		return -EINVAL;
 	if (mop->size > MEM_OP_MAX_SIZE)
@@ -2741,6 +2755,19 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
 	} else {
 		mop->key = 0;
 	}
+	if (mop->flags & KVM_S390_MEMOP_F_CMPXCHG) {
+		/*
+		 * This validates off_in_quad. Checking that size is a power
+		 * of two is not necessary, as cmpxchg_guest_abs_with_key
+		 * takes care of that
+		 */
+		if (mop->size > sizeof(new))
+			return -EINVAL;
+		if (copy_from_user(&new.raw[off_in_quad], uaddr, mop->size))
+			return -EFAULT;
+		if (copy_from_user(&old.raw[off_in_quad], old_addr, mop->size))
+			return -EFAULT;
+	}
 	if (!(mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY)) {
 		tmpbuf = vmalloc(mop->size);
 		if (!tmpbuf)
@@ -2771,6 +2798,14 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
 	case KVM_S390_MEMOP_ABSOLUTE_WRITE: {
 		if (mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY) {
 			r = check_gpa_range(kvm, mop->gaddr, mop->size, GACC_STORE, mop->key);
+		} else if (mop->flags & KVM_S390_MEMOP_F_CMPXCHG) {
+			r = cmpxchg_guest_abs_with_key(kvm, mop->gaddr, mop->size,
+						       &old.quad, new.quad, mop->key);
+			if (r == 1) {
+				r = KVM_S390_MEMOP_R_NO_XCHG;
+				if (copy_to_user(old_addr, &old.raw[off_in_quad], mop->size))
+					r = -EFAULT;
+			}
 		} else {
 			if (copy_from_user(tmpbuf, uaddr, mop->size)) {
 				r = -EFAULT;
