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- .. SPDX-License-Identifier: GPL-2.0
- Paravirtualized time support for arm64
- ======================================
- Arm specification DEN0057/A defines a standard for paravirtualised time
- support for AArch64 guests:
- https://developer.arm.com/docs/den0057/a
- KVM/arm64 implements the stolen time part of this specification by providing
- some hypervisor service calls to support a paravirtualized guest obtaining a
- view of the amount of time stolen from its execution.
- Two new SMCCC compatible hypercalls are defined:
- * PV_TIME_FEATURES: 0xC5000020
- * PV_TIME_ST: 0xC5000021
- These are only available in the SMC64/HVC64 calling convention as
- paravirtualized time is not available to 32 bit Arm guests. The existence of
- the PV_TIME_FEATURES hypercall should be probed using the SMCCC 1.1
- ARCH_FEATURES mechanism before calling it.
- PV_TIME_FEATURES
- ============= ======== ==========
- Function ID: (uint32) 0xC5000020
- PV_call_id: (uint32) The function to query for support.
- Currently only PV_TIME_ST is supported.
- Return value: (int64) NOT_SUPPORTED (-1) or SUCCESS (0) if the relevant
- PV-time feature is supported by the hypervisor.
- ============= ======== ==========
- PV_TIME_ST
- ============= ======== ==========
- Function ID: (uint32) 0xC5000021
- Return value: (int64) IPA of the stolen time data structure for this
- VCPU. On failure:
- NOT_SUPPORTED (-1)
- ============= ======== ==========
- The IPA returned by PV_TIME_ST should be mapped by the guest as normal memory
- with inner and outer write back caching attributes, in the inner shareable
- domain. A total of 16 bytes from the IPA returned are guaranteed to be
- meaningfully filled by the hypervisor (see structure below).
- PV_TIME_ST returns the structure for the calling VCPU.
- Stolen Time
- -----------
- The structure pointed to by the PV_TIME_ST hypercall is as follows:
- +-------------+-------------+-------------+----------------------------+
- | Field | Byte Length | Byte Offset | Description |
- +=============+=============+=============+============================+
- | Revision | 4 | 0 | Must be 0 for version 1.0 |
- +-------------+-------------+-------------+----------------------------+
- | Attributes | 4 | 4 | Must be 0 |
- +-------------+-------------+-------------+----------------------------+
- | Stolen time | 8 | 8 | Stolen time in unsigned |
- | | | | nanoseconds indicating how |
- | | | | much time this VCPU thread |
- | | | | was involuntarily not |
- | | | | running on a physical CPU. |
- +-------------+-------------+-------------+----------------------------+
- All values in the structure are stored little-endian.
- The structure will be updated by the hypervisor prior to scheduling a VCPU. It
- will be present within a reserved region of the normal memory given to the
- guest. The guest should not attempt to write into this memory. There is a
- structure per VCPU of the guest.
- It is advisable that one or more 64k pages are set aside for the purpose of
- these structures and not used for other purposes, this enables the guest to map
- the region using 64k pages and avoids conflicting attributes with other memory.
- For the user space interface see Documentation/virt/kvm/devices/vcpu.rst
- section "3. GROUP: KVM_ARM_VCPU_PVTIME_CTRL".
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