1 .. SPDX-License-Identifier: GPL-2.0
2
3 =============================
4 The s390 DIAGNOSE call on KVM
5 =============================
6
7 KVM on s390 supports the DIAGNOSE call for mak
8 native hypercalls and for selected hypercalls
9 hypervisors.
10
11 Note that bits are numbered as by the usual s3
12 bit on the left).
13
14
15 General remarks
16 ---------------
17
18 DIAGNOSE calls by the guest cause a mandatory
19 all supported DIAGNOSE calls need to be handle
20 userspace.
21
22 All DIAGNOSE calls supported by KVM use the RS
23
24 --------------------------------------
25 | '83' | R1 | R3 | B2 | D2 |
26 --------------------------------------
27 0 8 12 16 20 31
28
29 The second-operand address (obtained by the ba
30 is not used to address data. Instead, bits 48-
31 the function code, and bits 0-47 are ignored.
32
33 The supported DIAGNOSE function codes vary by
34 DIAGNOSE function codes not specific to KVM, p
35 documentation for the s390 hypervisors definin
36
37
38 DIAGNOSE function code 'X'500' - KVM virtio fu
39 ----------------------------------------------
40
41 If the function code specifies 0x500, various
42 are performed.
43
44 General register 1 contains the virtio subfunc
45 virtio subfunctions depend on KVM's userspace.
46 provides either s390-virtio (subcodes 0-2) or
47
48 Upon completion of the DIAGNOSE instruction, g
49 the function's return code, which is either a
50 specific value.
51
52 Subcode 0 - s390-virtio notification and early
53 Handled by userspace.
54
55 Subcode 1 - s390-virtio reset
56 Handled by userspace.
57
58 Subcode 2 - s390-virtio set status
59 Handled by userspace.
60
61 Subcode 3 - virtio-ccw notification
62 Handled by either userspace or KVM (ioeven
63
64 General register 2 contains a subchannel-i
65 the subchannel of the virtio-ccw proxy dev
66
67 General register 3 contains the number of
68
69 General register 4 contains a 64bit identi
70 kvm_io_bus cookie). If general register 4
71 identifier, it is ignored.
72
73 After completion of the DIAGNOSE call, gen
74 a 64bit identifier (in the kvm_io_bus cook
75 error value, if an internal error occurred
76
77 See also the virtio standard for a discuss
78
79
80 DIAGNOSE function code 'X'501 - KVM breakpoint
81 ----------------------------------------------
82
83 If the function code specifies 0x501, breakpoi
84 This function code is handled by userspace.
85
86 This diagnose function code has no subfunction
87
88
89 DIAGNOSE function code 'X'9C - Voluntary Time
90 ----------------------------------------------
91
92 General register 1 contains the target CPU add
93
94 In a guest of a hypervisor like LPAR, KVM or z
95 DIAGNOSE with function code 0x9c may improve s
96 yielding the host CPU on which the guest CPU i
97 to another guest CPU, preferably the logical C
98 target CPU.
99
100
101 DIAG 'X'9C forwarding
102 +++++++++++++++++++++
103
104 The guest may send a DIAGNOSE 0x9c in order to
105 other vcpu. An example is a Linux guest that t
106 that is currently holding a spinlock, but not
107
108 However, on the host the real cpu backing the
109 running.
110 Forwarding the DIAGNOSE 0x9c initially sent by
111 the backing cpu will hopefully cause that cpu,
112 the guest's vcpu, to be scheduled.
113
114
115 diag9c_forwarding_hz
116 KVM kernel parameter allowing to specify t
117 0x9c forwarding per second in the purpose
118 forwarding storm.
119 A value of 0 turns the forwarding off.
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