From d824ca52abd020a36948d12f2c6704ea2ae12513 Mon Sep 17 00:00:00 2001 From: Marc Zyngier Date: Mon, 3 Apr 2017 19:38:07 +0100 Subject: arm/arm64: Add hyp-stub API documentation In order to help people understanding the hyp-stub API that exists between the host kernel and the hypervisor mode (whether a hypervisor has been installed or not), let's document said API. As with any form of documentation, I expect it to become obsolete and completely misleading within 20 minutes after having being merged. Acked-by: Russell King Acked-by: Catalin Marinas Signed-off-by: Marc Zyngier Signed-off-by: Christoffer Dall --- Documentation/virtual/kvm/arm/hyp-abi.txt | 53 +++++++++++++++++++++++++++++++ 1 file changed, 53 insertions(+) create mode 100644 Documentation/virtual/kvm/arm/hyp-abi.txt (limited to 'Documentation/virtual') diff --git a/Documentation/virtual/kvm/arm/hyp-abi.txt b/Documentation/virtual/kvm/arm/hyp-abi.txt new file mode 100644 index 000000000000..a20a0bee268d --- /dev/null +++ b/Documentation/virtual/kvm/arm/hyp-abi.txt @@ -0,0 +1,53 @@ +* Internal ABI between the kernel and HYP + +This file documents the interaction between the Linux kernel and the +hypervisor layer when running Linux as a hypervisor (for example +KVM). It doesn't cover the interaction of the kernel with the +hypervisor when running as a guest (under Xen, KVM or any other +hypervisor), or any hypervisor-specific interaction when the kernel is +used as a host. + +On arm and arm64 (without VHE), the kernel doesn't run in hypervisor +mode, but still needs to interact with it, allowing a built-in +hypervisor to be either installed or torn down. + +In order to achieve this, the kernel must be booted at HYP (arm) or +EL2 (arm64), allowing it to install a set of stubs before dropping to +SVC/EL1. These stubs are accessible by using a 'hvc #0' instruction, +and only act on individual CPUs. + +Unless specified otherwise, any built-in hypervisor must implement +these functions (see arch/arm{,64}/include/asm/virt.h): + +* r0/x0 = HVC_SET_VECTORS + r1/x1 = vectors + + Set HVBAR/VBAR_EL2 to 'vectors' to enable a hypervisor. 'vectors' + must be a physical address, and respect the alignment requirements + of the architecture. Only implemented by the initial stubs, not by + Linux hypervisors. + +* r0/x0 = HVC_RESET_VECTORS + + Turn HYP/EL2 MMU off, and reset HVBAR/VBAR_EL2 to the initials + stubs' exception vector value. This effectively disables an existing + hypervisor. + +* r0/x0 = HVC_SOFT_RESTART + r1/x1 = restart address + x2 = x0's value when entering the next payload (arm64) + x3 = x1's value when entering the next payload (arm64) + x4 = x2's value when entering the next payload (arm64) + + Mask all exceptions, disable the MMU, move the arguments into place + (arm64 only), and jump to the restart address while at HYP/EL2. This + hypercall is not expected to return to its caller. + +Any other value of r0/x0 triggers a hypervisor-specific handling, +which is not documented here. + +The return value of a stub hypercall is held by r0/x0, and is 0 on +success, and HVC_STUB_ERR on error. A stub hypercall is allowed to +clobber any of the caller-saved registers (x0-x18 on arm64, r0-r3 and +ip on arm). It is thus recommended to use a function call to perform +the hypercall. -- cgit