XEN Hypercall IOCTLs (privcmd)

IMPLEMENTATION

Since ARM uses second stage paging any machine/physical addresses passed to hypercalls are Guest Physical Addresses (Intermediate Physical Addresses) unless otherwise noted.

• A hypercall is issued using the ARM HVC instruction.

• A hypercall can take up to 5 arguments. These are passed in registers, the first argument in x0/r0 (for arm64/arm32 guests respectively irrespective of whether the underlying hypervisor is 32- or 64-bit), the second argument in x1/r1, the third in x2/r2, the forth in x3/r3 and the fifth in x4/r4.

• The hypercall number is passed in r12 (arm) or x16 (arm64). In both cases the relevant ARM procedure calling convention specifies this is an inter-procedure-call scratch register (e.g. for use in linker stubs). This use does not conflict with use during a hypercall.

• The HVC ISS must contain a Xen specific TAG: XEN_HYPERCALL_TAG.

• The return value is in x0/r0.

• The hypercall will clobber x16/r12 and the argument registers used by that hypercall (except r0 which is the return value) i.e. in addition to x16/r12 a 2 argument hypercall will clobber x1/r1 and a 4 argument hypercall will clobber x1/r1, x2/r2 and x3/r3.

• Parameter structs passed to hypercalls are laid out according to the Procedure Call Standard for the ARM Architecture (AAPCS, AKA EABI) and Procedure Call Standard for the ARM 64-bit Architecture (AAPCS64). Where there is a conflict the 64-bit standard should be used regardless of guest type. Structures which are passed as hypercall arguments are always little endian.

• All memory which is shared with other entities in the system (including the hypervisor and other guests) must reside in memory which is mapped as Normal Inner Write-Back Outer Write-Back Inner-Shareable. This applies to:

  • hypercall arguments passed via a pointer to guest memory.
  • memory shared via the grant table mechanism (including PV I/O rings etc).
  • memory shared with the hypervisor (struct shared_info, struct vcpu_info, the grant table, etc).

• Any cache allocation hints are acceptable.

https://xenbits.xen.org/docs/unstable/hypercall/arm/include,public,arch-arm.h.html#incontents_arm_abi

https://elixir.bootlin.com/linux/v4.16.7/source/drivers/xen/privcmd.c#L859

Xen Interface - Page 17

http://www-archive.xenproject.org/files/xen_interface.pdf

https://xenbits.xen.org/docs/unstable/hypercall/index.html

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