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Documentation / virtual / kvm / s390-diag.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1	The s390 DIAGNOSE call on KVM
2	=============================
3	
4	KVM on s390 supports the DIAGNOSE call for making hypercalls, both for
5	native hypercalls and for selected hypercalls found on other s390
6	hypervisors.
7	
8	Note that bits are numbered as by the usual s390 convention (most significant
9	bit on the left).
10	
11	
12	General remarks
13	---------------
14	
15	DIAGNOSE calls by the guest cause a mandatory intercept. This implies
16	all supported DIAGNOSE calls need to be handled by either KVM or its
17	userspace.
18	
19	All DIAGNOSE calls supported by KVM use the RS-a format:
20	
21	--------------------------------------
22	|  '83'  | R1 | R3 | B2 |     D2     |
23	--------------------------------------
24	0        8    12   16   20           31
25	
26	The second-operand address (obtained by the base/displacement calculation)
27	is not used to address data. Instead, bits 48-63 of this address specify
28	the function code, and bits 0-47 are ignored.
29	
30	The supported DIAGNOSE function codes vary by the userspace used. For
31	DIAGNOSE function codes not specific to KVM, please refer to the
32	documentation for the s390 hypervisors defining them.
33	
34	
35	DIAGNOSE function code 'X'500' - KVM virtio functions
36	-----------------------------------------------------
37	
38	If the function code specifies 0x500, various virtio-related functions
39	are performed.
40	
41	General register 1 contains the virtio subfunction code. Supported
42	virtio subfunctions depend on KVM's userspace. Generally, userspace
43	provides either s390-virtio (subcodes 0-2) or virtio-ccw (subcode 3).
44	
45	Upon completion of the DIAGNOSE instruction, general register 2 contains
46	the function's return code, which is either a return code or a subcode
47	specific value.
48	
49	Subcode 0 - s390-virtio notification and early console printk
50	    Handled by userspace.
51	
52	Subcode 1 - s390-virtio reset
53	    Handled by userspace.
54	
55	Subcode 2 - s390-virtio set status
56	    Handled by userspace.
57	
58	Subcode 3 - virtio-ccw notification
59	    Handled by either userspace or KVM (ioeventfd case).
60	
61	    General register 2 contains a subchannel-identification word denoting
62	    the subchannel of the virtio-ccw proxy device to be notified.
63	
64	    General register 3 contains the number of the virtqueue to be notified.
65	
66	    General register 4 contains a 64bit identifier for KVM usage (the
67	    kvm_io_bus cookie). If general register 4 does not contain a valid
68	    identifier, it is ignored.
69	
70	    After completion of the DIAGNOSE call, general register 2 may contain
71	    a 64bit identifier (in the kvm_io_bus cookie case).
72	
73	    See also the virtio standard for a discussion of this hypercall.
74	
75	
76	DIAGNOSE function code 'X'501 - KVM breakpoint
77	----------------------------------------------
78	
79	If the function code specifies 0x501, breakpoint functions may be performed.
80	This function code is handled by userspace.
81	
82	This diagnose function code has no subfunctions and uses no parameters.
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