s390-diag.rst

  1.. SPDX-License-Identifier: GPL-2.0
  2
  3=============================
  4The s390 DIAGNOSE call on KVM
  5=============================
  6
  7KVM on s390 supports the DIAGNOSE call for making hypercalls, both for
  8native hypercalls and for selected hypercalls found on other s390
  9hypervisors.
 10
 11Note that bits are numbered as by the usual s390 convention (most significant
 12bit on the left).
 13
 14
 15General remarks
 16---------------
 17
 18DIAGNOSE calls by the guest cause a mandatory intercept. This implies
 19all supported DIAGNOSE calls need to be handled by either KVM or its
 20userspace.
 21
 22All DIAGNOSE calls supported by KVM use the RS-a format::
 23
 24  --------------------------------------
 25  |  '83'  | R1 | R3 | B2 |     D2     |
 26  --------------------------------------
 27  0        8    12   16   20           31
 28
 29The second-operand address (obtained by the base/displacement calculation)
 30is not used to address data. Instead, bits 48-63 of this address specify
 31the function code, and bits 0-47 are ignored.
 32
 33The supported DIAGNOSE function codes vary by the userspace used. For
 34DIAGNOSE function codes not specific to KVM, please refer to the
 35documentation for the s390 hypervisors defining them.
 36
 37
 38DIAGNOSE function code 'X'500' - KVM functions
 39----------------------------------------------
 40
 41If the function code specifies 0x500, various KVM-specific functions
 42are performed, including virtio functions.
 43
 44General register 1 contains the subfunction code. Supported subfunctions
 45depend on KVM's userspace. Regarding virtio subfunctions, generally
 46userspace provides either s390-virtio (subcodes 0-2) or virtio-ccw
 47(subcode 3).
 48
 49Upon completion of the DIAGNOSE instruction, general register 2 contains
 50the function's return code, which is either a return code or a subcode
 51specific value.
 52
 53If the specified subfunction is not supported, a SPECIFICATION exception
 54will be triggered.
 55
 56Subcode 0 - s390-virtio notification and early console printk
 57    Handled by userspace.
 58
 59Subcode 1 - s390-virtio reset
 60    Handled by userspace.
 61
 62Subcode 2 - s390-virtio set status
 63    Handled by userspace.
 64
 65Subcode 3 - virtio-ccw notification
 66    Handled by either userspace or KVM (ioeventfd case).
 67
 68    General register 2 contains a subchannel-identification word denoting
 69    the subchannel of the virtio-ccw proxy device to be notified.
 70
 71    General register 3 contains the number of the virtqueue to be notified.
 72
 73    General register 4 contains a 64bit identifier for KVM usage (the
 74    kvm_io_bus cookie). If general register 4 does not contain a valid
 75    identifier, it is ignored.
 76
 77    After completion of the DIAGNOSE call, general register 2 may contain
 78    a 64bit identifier (in the kvm_io_bus cookie case), or a negative
 79    error value, if an internal error occurred.
 80
 81    See also the virtio standard for a discussion of this hypercall.
 82
 83Subcode 4 - storage-limit
 84    Handled by userspace.
 85
 86    After completion of the DIAGNOSE call, general register 2 will
 87    contain the storage limit: the maximum physical address that might be
 88    used for storage throughout the lifetime of the VM.
 89
 90    The storage limit does not indicate currently usable storage, it may
 91    include holes, standby storage and areas reserved for other means, such
 92    as memory hotplug or virtio-mem devices. Other interfaces for detecting
 93    actually usable storage, such as SCLP, must be used in conjunction with
 94    this subfunction.
 95
 96    Note that the storage limit can be larger, but never smaller than the
 97    maximum storage address indicated by SCLP via the "maximum storage
 98    increment" and the "increment size".
 99
100
101DIAGNOSE function code 'X'501 - KVM breakpoint
102----------------------------------------------
103
104If the function code specifies 0x501, breakpoint functions may be performed.
105This function code is handled by userspace.
106
107This diagnose function code has no subfunctions and uses no parameters.
108
109
110DIAGNOSE function code 'X'9C - Voluntary Time Slice Yield
111---------------------------------------------------------
112
113General register 1 contains the target CPU address.
114
115In a guest of a hypervisor like LPAR, KVM or z/VM using shared host CPUs,
116DIAGNOSE with function code 0x9c may improve system performance by
117yielding the host CPU on which the guest CPU is running to be assigned
118to another guest CPU, preferably the logical CPU containing the specified
119target CPU.
120
121
122DIAG 'X'9C forwarding
123+++++++++++++++++++++
124
125The guest may send a DIAGNOSE 0x9c in order to yield to a certain
126other vcpu. An example is a Linux guest that tries to yield to the vcpu
127that is currently holding a spinlock, but not running.
128
129However, on the host the real cpu backing the vcpu may itself not be
130running.
131Forwarding the DIAGNOSE 0x9c initially sent by the guest to yield to
132the backing cpu will hopefully cause that cpu, and thus subsequently
133the guest's vcpu, to be scheduled.
134
135
136diag9c_forwarding_hz
137    KVM kernel parameter allowing to specify the maximum number of DIAGNOSE
138    0x9c forwarding per second in the purpose of avoiding a DIAGNOSE 0x9c
139    forwarding storm.
140    A value of 0 turns the forwarding off.