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