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+When used with the "pseries" machine type, QEMU-system-ppc64 implements
+a set of hypervisor calls using a subset of the server "PAPR" specification
+(IBM internal at this point), which is also what IBM's proprietary hypervisor
+adheres too.
+
+The subset is selected based on the requirements of Linux as a guest.
+
+In addition to those calls, we have added our own private hypervisor
+calls which are mostly used as a private interface between the firmware
+running in the guest and QEMU.
+
+All those hypercalls start at hcall number 0xf000 which correspond
+to a implementation specific range in PAPR.
+
+- H_RTAS (0xf000)
+
+RTAS is a set of runtime services generally provided by the firmware
+inside the guest to the operating system. It predates the existence
+of hypervisors (it was originally an extension to Open Firmware) and
+is still used by PAPR to provide various services that aren't performance
+sensitive.
+
+We currently implement the RTAS services in QEMU itself. The actual RTAS
+"firmware" blob in the guest is a small stub of a few instructions which
+calls our private H_RTAS hypervisor call to pass the RTAS calls to QEMU.
+
+Arguments:
+
+  r3 : H_RTAS (0xf000)
+  r4 : Guest physical address of RTAS parameter block
+
+Returns:
+
+  H_SUCCESS   : Successfully called the RTAS function (RTAS result
+                will have been stored in the parameter block)
+  H_PARAMETER : Unknown token
+
+- H_LOGICAL_MEMOP (0xf001)
+
+When the guest runs in "real mode" (in powerpc lingua this means
+with MMU disabled, ie guest effective == guest physical), it only
+has access to a subset of memory and no IOs.
+
+PAPR provides a set of hypervisor calls to perform cacheable or
+non-cacheable accesses to any guest physical addresses that the
+guest can use in order to access IO devices while in real mode.
+
+This is typically used by the firmware running in the guest.
+
+However, doing a hypercall for each access is extremely inefficient
+(even more so when running KVM) when accessing the frame buffer. In
+that case, things like scrolling become unusably slow.
+
+This hypercall allows the guest to request a "memory op" to be applied
+to memory. The supported memory ops at this point are to copy a range
+of memory (supports overlap of source and destination) and XOR which
+is used by our SLOF firmware to invert the screen.
+
+Arguments:
+
+  r3: H_LOGICAL_MEMOP (0xf001)
+  r4: Guest physical address of destination
+  r5: Guest physical address of source
+  r6: Individual element size
+        0 = 1 byte
+        1 = 2 bytes
+        2 = 4 bytes
+        3 = 8 bytes
+  r7: Number of elements
+  r8: Operation
+        0 = copy
+        1 = xor
+
+Returns:
+
+  H_SUCCESS   : Success
+  H_PARAMETER : Invalid argument
+