--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+
+===================
+Linux KVM Hypercall
+===================
+
+X86:
+ KVM Hypercalls have a three-byte sequence of either the vmcall or the vmmcall
+ instruction. The hypervisor can replace it with instructions that are
+ guaranteed to be supported.
+
+ Up to four arguments may be passed in rbx, rcx, rdx, and rsi respectively.
+ The hypercall number should be placed in rax and the return value will be
+ placed in rax. No other registers will be clobbered unless explicitly stated
+ by the particular hypercall.
+
+S390:
+ R2-R7 are used for parameters 1-6. In addition, R1 is used for hypercall
+ number. The return value is written to R2.
+
+ S390 uses diagnose instruction as hypercall (0x500) along with hypercall
+ number in R1.
+
+ For further information on the S390 diagnose call as supported by KVM,
+ refer to Documentation/virt/kvm/s390-diag.rst.
+
+PowerPC:
+ It uses R3-R10 and hypercall number in R11. R4-R11 are used as output registers.
+ Return value is placed in R3.
+
+ KVM hypercalls uses 4 byte opcode, that are patched with 'hypercall-instructions'
+ property inside the device tree's /hypervisor node.
+ For more information refer to Documentation/virt/kvm/ppc-pv.rst
+
+MIPS:
+ KVM hypercalls use the HYPCALL instruction with code 0 and the hypercall
+ number in $2 (v0). Up to four arguments may be placed in $4-$7 (a0-a3) and
+ the return value is placed in $2 (v0).
+
+KVM Hypercalls Documentation
+============================
+
+The template for each hypercall is:
+1. Hypercall name.
+2. Architecture(s)
+3. Status (deprecated, obsolete, active)
+4. Purpose
+
+1. KVM_HC_VAPIC_POLL_IRQ
+------------------------
+
+:Architecture: x86
+:Status: active
+:Purpose: Trigger guest exit so that the host can check for pending
+ interrupts on reentry.
+
+2. KVM_HC_MMU_OP
+----------------
+
+:Architecture: x86
+:Status: deprecated.
+:Purpose: Support MMU operations such as writing to PTE,
+ flushing TLB, release PT.
+
+3. KVM_HC_FEATURES
+------------------
+
+:Architecture: PPC
+:Status: active
+:Purpose: Expose hypercall availability to the guest. On x86 platforms, cpuid
+ used to enumerate which hypercalls are available. On PPC, either
+ device tree based lookup ( which is also what EPAPR dictates)
+ OR KVM specific enumeration mechanism (which is this hypercall)
+ can be used.
+
+4. KVM_HC_PPC_MAP_MAGIC_PAGE
+----------------------------
+
+:Architecture: PPC
+:Status: active
+:Purpose: To enable communication between the hypervisor and guest there is a
+ shared page that contains parts of supervisor visible register state.
+ The guest can map this shared page to access its supervisor register
+ through memory using this hypercall.
+
+5. KVM_HC_KICK_CPU
+------------------
+
+:Architecture: x86
+:Status: active
+:Purpose: Hypercall used to wakeup a vcpu from HLT state
+:Usage example:
+ A vcpu of a paravirtualized guest that is busywaiting in guest
+ kernel mode for an event to occur (ex: a spinlock to become available) can
+ execute HLT instruction once it has busy-waited for more than a threshold
+ time-interval. Execution of HLT instruction would cause the hypervisor to put
+ the vcpu to sleep until occurrence of an appropriate event. Another vcpu of the
+ same guest can wakeup the sleeping vcpu by issuing KVM_HC_KICK_CPU hypercall,
+ specifying APIC ID (a1) of the vcpu to be woken up. An additional argument (a0)
+ is used in the hypercall for future use.
+
+
+6. KVM_HC_CLOCK_PAIRING
+-----------------------
+:Architecture: x86
+:Status: active
+:Purpose: Hypercall used to synchronize host and guest clocks.
+
+Usage:
+
+a0: guest physical address where host copies
+"struct kvm_clock_offset" structure.
+
+a1: clock_type, ATM only KVM_CLOCK_PAIRING_WALLCLOCK (0)
+is supported (corresponding to the host's CLOCK_REALTIME clock).
+
+ ::
+
+ struct kvm_clock_pairing {
+ __s64 sec;
+ __s64 nsec;
+ __u64 tsc;
+ __u32 flags;
+ __u32 pad[9];
+ };
+
+ Where:
+ * sec: seconds from clock_type clock.
+ * nsec: nanoseconds from clock_type clock.
+ * tsc: guest TSC value used to calculate sec/nsec pair
+ * flags: flags, unused (0) at the moment.
+
+The hypercall lets a guest compute a precise timestamp across
+host and guest. The guest can use the returned TSC value to
+compute the CLOCK_REALTIME for its clock, at the same instant.
+
+Returns KVM_EOPNOTSUPP if the host does not use TSC clocksource,
+or if clock type is different than KVM_CLOCK_PAIRING_WALLCLOCK.
+
+6. KVM_HC_SEND_IPI
+------------------
+
+:Architecture: x86
+:Status: active
+:Purpose: Send IPIs to multiple vCPUs.
+
+- a0: lower part of the bitmap of destination APIC IDs
+- a1: higher part of the bitmap of destination APIC IDs
+- a2: the lowest APIC ID in bitmap
+- a3: APIC ICR
+
+The hypercall lets a guest send multicast IPIs, with at most 128
+128 destinations per hypercall in 64-bit mode and 64 vCPUs per
+hypercall in 32-bit mode. The destinations are represented by a
+bitmap contained in the first two arguments (a0 and a1). Bit 0 of
+a0 corresponds to the APIC ID in the third argument (a2), bit 1
+corresponds to the APIC ID a2+1, and so on.
+
+Returns the number of CPUs to which the IPIs were delivered successfully.
+
+7. KVM_HC_SCHED_YIELD
+---------------------
+
+:Architecture: x86
+:Status: active
+:Purpose: Hypercall used to yield if the IPI target vCPU is preempted
+
+a0: destination APIC ID
+
+:Usage example: When sending a call-function IPI-many to vCPUs, yield if
+ any of the IPI target vCPUs was preempted.
+
+8. KVM_HC_MAP_GPA_RANGE
+-------------------------
+:Architecture: x86
+:Status: active
+:Purpose: Request KVM to map a GPA range with the specified attributes.
+
+a0: the guest physical address of the start page
+a1: the number of (4kb) pages (must be contiguous in GPA space)
+a2: attributes
+
+ Where 'attributes' :
+ * bits 3:0 - preferred page size encoding 0 = 4kb, 1 = 2mb, 2 = 1gb, etc...
+ * bit 4 - plaintext = 0, encrypted = 1
+ * bits 63:5 - reserved (must be zero)
+
+**Implementation note**: this hypercall is implemented in userspace via
+the KVM_CAP_EXIT_HYPERCALL capability. Userspace must enable that capability
+before advertising KVM_FEATURE_HC_MAP_GPA_RANGE in the guest CPUID. In
+addition, if the guest supports KVM_FEATURE_MIGRATION_CONTROL, userspace
+must also set up an MSR filter to process writes to MSR_KVM_MIGRATION_CONTROL.
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