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+.. SPDX-License-Identifier: GPL-2.0
+
+=============================
+The s390 DIAGNOSE call on KVM
+=============================
+
+KVM on s390 supports the DIAGNOSE call for making hypercalls, both for
+native hypercalls and for selected hypercalls found on other s390
+hypervisors.
+
+Note that bits are numbered as by the usual s390 convention (most significant
+bit on the left).
+
+
+General remarks
+---------------
+
+DIAGNOSE calls by the guest cause a mandatory intercept. This implies
+all supported DIAGNOSE calls need to be handled by either KVM or its
+userspace.
+
+All DIAGNOSE calls supported by KVM use the RS-a format::
+
+  --------------------------------------
+  |  '83'  | R1 | R3 | B2 |     D2     |
+  --------------------------------------
+  0        8    12   16   20           31
+
+The second-operand address (obtained by the base/displacement calculation)
+is not used to address data. Instead, bits 48-63 of this address specify
+the function code, and bits 0-47 are ignored.
+
+The supported DIAGNOSE function codes vary by the userspace used. For
+DIAGNOSE function codes not specific to KVM, please refer to the
+documentation for the s390 hypervisors defining them.
+
+
+DIAGNOSE function code 'X'500' - KVM virtio functions
+-----------------------------------------------------
+
+If the function code specifies 0x500, various virtio-related functions
+are performed.
+
+General register 1 contains the virtio subfunction code. Supported
+virtio subfunctions depend on KVM's userspace. Generally, userspace
+provides either s390-virtio (subcodes 0-2) or virtio-ccw (subcode 3).
+
+Upon completion of the DIAGNOSE instruction, general register 2 contains
+the function's return code, which is either a return code or a subcode
+specific value.
+
+Subcode 0 - s390-virtio notification and early console printk
+    Handled by userspace.
+
+Subcode 1 - s390-virtio reset
+    Handled by userspace.
+
+Subcode 2 - s390-virtio set status
+    Handled by userspace.
+
+Subcode 3 - virtio-ccw notification
+    Handled by either userspace or KVM (ioeventfd case).
+
+    General register 2 contains a subchannel-identification word denoting
+    the subchannel of the virtio-ccw proxy device to be notified.
+
+    General register 3 contains the number of the virtqueue to be notified.
+
+    General register 4 contains a 64bit identifier for KVM usage (the
+    kvm_io_bus cookie). If general register 4 does not contain a valid
+    identifier, it is ignored.
+
+    After completion of the DIAGNOSE call, general register 2 may contain
+    a 64bit identifier (in the kvm_io_bus cookie case), or a negative
+    error value, if an internal error occurred.
+
+    See also the virtio standard for a discussion of this hypercall.
+
+
+DIAGNOSE function code 'X'501 - KVM breakpoint
+----------------------------------------------
+
+If the function code specifies 0x501, breakpoint functions may be performed.
+This function code is handled by userspace.
+
+This diagnose function code has no subfunctions and uses no parameters.
+
+
+DIAGNOSE function code 'X'9C - Voluntary Time Slice Yield
+---------------------------------------------------------
+
+General register 1 contains the target CPU address.
+
+In a guest of a hypervisor like LPAR, KVM or z/VM using shared host CPUs,
+DIAGNOSE with function code 0x9c may improve system performance by
+yielding the host CPU on which the guest CPU is running to be assigned
+to another guest CPU, preferably the logical CPU containing the specified
+target CPU.
+
+
+DIAG 'X'9C forwarding
++++++++++++++++++++++
+
+The guest may send a DIAGNOSE 0x9c in order to yield to a certain
+other vcpu. An example is a Linux guest that tries to yield to the vcpu
+that is currently holding a spinlock, but not running.
+
+However, on the host the real cpu backing the vcpu may itself not be
+running.
+Forwarding the DIAGNOSE 0x9c initially sent by the guest to yield to
+the backing cpu will hopefully cause that cpu, and thus subsequently
+the guest's vcpu, to be scheduled.
+
+
+diag9c_forwarding_hz
+    KVM kernel parameter allowing to specify the maximum number of DIAGNOSE
+    0x9c forwarding per second in the purpose of avoiding a DIAGNOSE 0x9c
+    forwarding storm.
+    A value of 0 turns the forwarding off.