78 lines
2.8 KiB
ReStructuredText
78 lines
2.8 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=======================================
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Internal ABI between the kernel and HYP
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=======================================
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This file documents the interaction between the Linux kernel and the
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hypervisor layer when running Linux as a hypervisor (for example
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KVM). It doesn't cover the interaction of the kernel with the
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hypervisor when running as a guest (under Xen, KVM or any other
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hypervisor), or any hypervisor-specific interaction when the kernel is
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used as a host.
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Note: KVM/arm has been removed from the kernel. The API described
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here is still valid though, as it allows the kernel to kexec when
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booted at HYP. It can also be used by a hypervisor other than KVM
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if necessary.
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On arm and arm64 (without VHE), the kernel doesn't run in hypervisor
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mode, but still needs to interact with it, allowing a built-in
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hypervisor to be either installed or torn down.
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In order to achieve this, the kernel must be booted at HYP (arm) or
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EL2 (arm64), allowing it to install a set of stubs before dropping to
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SVC/EL1. These stubs are accessible by using a 'hvc #0' instruction,
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and only act on individual CPUs.
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Unless specified otherwise, any built-in hypervisor must implement
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these functions (see arch/arm{,64}/include/asm/virt.h):
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* ::
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r0/x0 = HVC_SET_VECTORS
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r1/x1 = vectors
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Set HVBAR/VBAR_EL2 to 'vectors' to enable a hypervisor. 'vectors'
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must be a physical address, and respect the alignment requirements
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of the architecture. Only implemented by the initial stubs, not by
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Linux hypervisors.
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* ::
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r0/x0 = HVC_RESET_VECTORS
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Turn HYP/EL2 MMU off, and reset HVBAR/VBAR_EL2 to the initials
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stubs' exception vector value. This effectively disables an existing
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hypervisor.
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* ::
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r0/x0 = HVC_SOFT_RESTART
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r1/x1 = restart address
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x2 = x0's value when entering the next payload (arm64)
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x3 = x1's value when entering the next payload (arm64)
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x4 = x2's value when entering the next payload (arm64)
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Mask all exceptions, disable the MMU, clear I+D bits, move the arguments
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into place (arm64 only), and jump to the restart address while at HYP/EL2.
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This hypercall is not expected to return to its caller.
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* ::
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x0 = HVC_VHE_RESTART (arm64 only)
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Attempt to upgrade the kernel's exception level from EL1 to EL2 by enabling
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the VHE mode. This is conditioned by the CPU supporting VHE, the EL2 MMU
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being off, and VHE not being disabled by any other means (command line
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option, for example).
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Any other value of r0/x0 triggers a hypervisor-specific handling,
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which is not documented here.
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The return value of a stub hypercall is held by r0/x0, and is 0 on
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success, and HVC_STUB_ERR on error. A stub hypercall is allowed to
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clobber any of the caller-saved registers (x0-x18 on arm64, r0-r3 and
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ip on arm). It is thus recommended to use a function call to perform
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the hypercall.
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