1.. SPDX-License-Identifier: GPL-2.0
2
3=======================
4ARM Hypercall Interface
5=======================
6
7KVM handles the hypercall services as requested by the guests. New hypercall
8services are regularly made available by the ARM specification or by KVM (as
9vendor services) if they make sense from a virtualization point of view.
10
11This means that a guest booted on two different versions of KVM can observe
12two different "firmware" revisions. This could cause issues if a given guest
13is tied to a particular version of a hypercall service, or if a migration
14causes a different version to be exposed out of the blue to an unsuspecting
15guest.
16
17In order to remedy this situation, KVM exposes a set of "firmware
18pseudo-registers" that can be manipulated using the GET/SET_ONE_REG
19interface. These registers can be saved/restored by userspace, and set
20to a convenient value as required.
21
22The following registers are defined:
23
24* KVM_REG_ARM_PSCI_VERSION:
25
26  KVM implements the PSCI (Power State Coordination Interface)
27  specification in order to provide services such as CPU on/off, reset
28  and power-off to the guest.
29
30  - Only valid if the vcpu has the KVM_ARM_VCPU_PSCI_0_2 feature set
31    (and thus has already been initialized)
32  - Returns the current PSCI version on GET_ONE_REG (defaulting to the
33    highest PSCI version implemented by KVM and compatible with v0.2)
34  - Allows any PSCI version implemented by KVM and compatible with
35    v0.2 to be set with SET_ONE_REG
36  - Affects the whole VM (even if the register view is per-vcpu)
37
38* KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1:
39    Holds the state of the firmware support to mitigate CVE-2017-5715, as
40    offered by KVM to the guest via a HVC call. The workaround is described
41    under SMCCC_ARCH_WORKAROUND_1 in [1].
42
43  Accepted values are:
44
45    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_NOT_AVAIL:
46      KVM does not offer
47      firmware support for the workaround. The mitigation status for the
48      guest is unknown.
49    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_AVAIL:
50      The workaround HVC call is
51      available to the guest and required for the mitigation.
52    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_1_NOT_REQUIRED:
53      The workaround HVC call
54      is available to the guest, but it is not needed on this VCPU.
55
56* KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2:
57    Holds the state of the firmware support to mitigate CVE-2018-3639, as
58    offered by KVM to the guest via a HVC call. The workaround is described
59    under SMCCC_ARCH_WORKAROUND_2 in [1]_.
60
61  Accepted values are:
62
63    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_AVAIL:
64      A workaround is not
65      available. KVM does not offer firmware support for the workaround.
66    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_UNKNOWN:
67      The workaround state is
68      unknown. KVM does not offer firmware support for the workaround.
69    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_AVAIL:
70      The workaround is available,
71      and can be disabled by a vCPU. If
72      KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_ENABLED is set, it is active for
73      this vCPU.
74    KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2_NOT_REQUIRED:
75      The workaround is always active on this vCPU or it is not needed.
76
77
78Bitmap Feature Firmware Registers
79---------------------------------
80
81Contrary to the above registers, the following registers exposes the
82hypercall services in the form of a feature-bitmap to the userspace. This
83bitmap is translated to the services that are available to the guest.
84There is a register defined per service call owner and can be accessed via
85GET/SET_ONE_REG interface.
86
87By default, these registers are set with the upper limit of the features
88that are supported. This way userspace can discover all the usable
89hypercall services via GET_ONE_REG. The user-space can write-back the
90desired bitmap back via SET_ONE_REG. The features for the registers that
91are untouched, probably because userspace isn't aware of them, will be
92exposed as is to the guest.
93
94Note that KVM will not allow the userspace to configure the registers
95anymore once any of the vCPUs has run at least once. Instead, it will
96return a -EBUSY.
97
98The pseudo-firmware bitmap register are as follows:
99
100* KVM_REG_ARM_STD_BMAP:
101    Controls the bitmap of the ARM Standard Secure Service Calls.
102
103  The following bits are accepted:
104
105    Bit-0: KVM_REG_ARM_STD_BIT_TRNG_V1_0:
106      The bit represents the services offered under v1.0 of ARM True Random
107      Number Generator (TRNG) specification, ARM DEN0098.
108
109* KVM_REG_ARM_STD_HYP_BMAP:
110    Controls the bitmap of the ARM Standard Hypervisor Service Calls.
111
112  The following bits are accepted:
113
114    Bit-0: KVM_REG_ARM_STD_HYP_BIT_PV_TIME:
115      The bit represents the Paravirtualized Time service as represented by
116      ARM DEN0057A.
117
118* KVM_REG_ARM_VENDOR_HYP_BMAP:
119    Controls the bitmap of the Vendor specific Hypervisor Service Calls.
120
121  The following bits are accepted:
122
123    Bit-0: KVM_REG_ARM_VENDOR_HYP_BIT_FUNC_FEAT
124      The bit represents the ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID
125      and ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID function-ids.
126
127    Bit-1: KVM_REG_ARM_VENDOR_HYP_BIT_PTP:
128      The bit represents the Precision Time Protocol KVM service.
129
130Errors:
131
132    =======  =============================================================
133    -ENOENT   Unknown register accessed.
134    -EBUSY    Attempt a 'write' to the register after the VM has started.
135    -EINVAL   Invalid bitmap written to the register.
136    =======  =============================================================
137
138.. [1] https://developer.arm.com/-/media/developer/pdf/ARM_DEN_0070A_Firmware_interfaces_for_mitigating_CVE-2017-5715.pdf
139