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Documentation / virtual / kvm / hypercalls.txt




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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

1	Linux KVM Hypercall:
2	===================
3	X86:
4	 KVM Hypercalls have a three-byte sequence of either the vmcall or the vmmcall
5	 instruction. The hypervisor can replace it with instructions that are
6	 guaranteed to be supported.
7	
8	 Up to four arguments may be passed in rbx, rcx, rdx, and rsi respectively.
9	 The hypercall number should be placed in rax and the return value will be
10	 placed in rax.  No other registers will be clobbered unless explicitly stated
11	 by the particular hypercall.
12	
13	S390:
14	  R2-R7 are used for parameters 1-6. In addition, R1 is used for hypercall
15	  number. The return value is written to R2.
16	
17	  S390 uses diagnose instruction as hypercall (0x500) along with hypercall
18	  number in R1.
19	
20	  For further information on the S390 diagnose call as supported by KVM,
21	  refer to Documentation/virtual/kvm/s390-diag.txt.
22	
23	 PowerPC:
24	  It uses R3-R10 and hypercall number in R11. R4-R11 are used as output registers.
25	  Return value is placed in R3.
26	
27	  KVM hypercalls uses 4 byte opcode, that are patched with 'hypercall-instructions'
28	  property inside the device tree's /hypervisor node.
29	  For more information refer to Documentation/virtual/kvm/ppc-pv.txt
30	
31	MIPS:
32	  KVM hypercalls use the HYPCALL instruction with code 0 and the hypercall
33	  number in $2 (v0). Up to four arguments may be placed in $4-$7 (a0-a3) and
34	  the return value is placed in $2 (v0).
35	
36	KVM Hypercalls Documentation
37	===========================
38	The template for each hypercall is:
39	1. Hypercall name.
40	2. Architecture(s)
41	3. Status (deprecated, obsolete, active)
42	4. Purpose
43	
44	1. KVM_HC_VAPIC_POLL_IRQ
45	------------------------
46	Architecture: x86
47	Status: active
48	Purpose: Trigger guest exit so that the host can check for pending
49	interrupts on reentry.
50	
51	2. KVM_HC_MMU_OP
52	------------------------
53	Architecture: x86
54	Status: deprecated.
55	Purpose: Support MMU operations such as writing to PTE,
56	flushing TLB, release PT.
57	
58	3. KVM_HC_FEATURES
59	------------------------
60	Architecture: PPC
61	Status: active
62	Purpose: Expose hypercall availability to the guest. On x86 platforms, cpuid
63	used to enumerate which hypercalls are available. On PPC, either device tree
64	based lookup ( which is also what EPAPR dictates) OR KVM specific enumeration
65	mechanism (which is this hypercall) can be used.
66	
67	4. KVM_HC_PPC_MAP_MAGIC_PAGE
68	------------------------
69	Architecture: PPC
70	Status: active
71	Purpose: To enable communication between the hypervisor and guest there is a
72	shared page that contains parts of supervisor visible register state.
73	The guest can map this shared page to access its supervisor register through
74	memory using this hypercall.
75	
76	5. KVM_HC_KICK_CPU
77	------------------------
78	Architecture: x86
79	Status: active
80	Purpose: Hypercall used to wakeup a vcpu from HLT state
81	Usage example : A vcpu of a paravirtualized guest that is busywaiting in guest
82	kernel mode for an event to occur (ex: a spinlock to become available) can
83	execute HLT instruction once it has busy-waited for more than a threshold
84	time-interval. Execution of HLT instruction would cause the hypervisor to put
85	the vcpu to sleep until occurrence of an appropriate event. Another vcpu of the
86	same guest can wakeup the sleeping vcpu by issuing KVM_HC_KICK_CPU hypercall,
87	specifying APIC ID (a1) of the vcpu to be woken up. An additional argument (a0)
88	is used in the hypercall for future use.
89	
90	
91	6. KVM_HC_CLOCK_PAIRING
92	------------------------
93	Architecture: x86
94	Status: active
95	Purpose: Hypercall used to synchronize host and guest clocks.
96	Usage:
97	
98	a0: guest physical address where host copies
99	"struct kvm_clock_offset" structure.
100	
101	a1: clock_type, ATM only KVM_CLOCK_PAIRING_WALLCLOCK (0)
102	is supported (corresponding to the host's CLOCK_REALTIME clock).
103	
104			struct kvm_clock_pairing {
105				__s64 sec;
106				__s64 nsec;
107				__u64 tsc;
108				__u32 flags;
109				__u32 pad[9];
110			};
111	
112	       Where:
113	               * sec: seconds from clock_type clock.
114	               * nsec: nanoseconds from clock_type clock.
115	               * tsc: guest TSC value used to calculate sec/nsec pair
116	               * flags: flags, unused (0) at the moment.
117	
118	The hypercall lets a guest compute a precise timestamp across
119	host and guest.  The guest can use the returned TSC value to
120	compute the CLOCK_REALTIME for its clock, at the same instant.
121	
122	Returns KVM_EOPNOTSUPP if the host does not use TSC clocksource,
123	or if clock type is different than KVM_CLOCK_PAIRING_WALLCLOCK.
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