[IA64] pvops: documentation on ia64/pv_ops

Paravirt_ops on IA64

====================

                          21 May 2008, Isaku Yamahata <yamahata@valinux.co.jp>

Introduction

------------

The aim of this documentation is to help with maintainability and/or to

encourage people to use paravirt_ops/IA64.

paravirt_ops (pv_ops in short) is a way for virtualization support of

Linux kernel on x86. Several ways for virtualization support were

proposed, paravirt_ops is the winner.

On the other hand, now there are also several IA64 virtualization

technologies like kvm/IA64, xen/IA64 and many other academic IA64

hypervisors so that it is good to add generic virtualization

infrastructure on Linux/IA64.

What is paravirt_ops?

---------------------

It has been developed on x86 as virtualization support via API, not ABI.

It allows each hypervisor to override operations which are important for

hypervisors at API level. And it allows a single kernel binary to run on

all supported execution environments including native machine.

Essentially paravirt_ops is a set of function pointers which represent

operations corresponding to low level sensitive instructions and high

level functionalities in various area. But one significant difference

from usual function pointer table is that it allows optimization with

binary patch. It is because some of these operations are very

performance sensitive and indirect call overhead is not negligible.

With binary patch, indirect C function call can be transformed into

direct C function call or in-place execution to eliminate the overhead.

Thus, operations of paravirt_ops are classified into three categories.

- simple indirect call

  These operations correspond to high level functionality so that the

  overhead of indirect call isn't very important.

- indirect call which allows optimization with binary patch

  Usually these operations correspond to low level instructions. They

  are called frequently and performance critical. So the overhead is

  very important.

- a set of macros for hand written assembly code

  Hand written assembly codes (.S files) also need paravirtualization

  because they include sensitive instructions or some of code paths in

  them are very performance critical.

The relation to the IA64 machine vector

---------------------------------------

Linux/IA64 has the IA64 machine vector functionality which allows the

kernel to switch implementations (e.g. initialization, ipi, dma api...)

depending on executing platform.

We can replace some implementations very easily defining a new machine

vector. Thus another approach for virtualization support would be

enhancing the machine vector functionality.

But paravirt_ops approach was taken because

- virtualization support needs wider support than machine vector does.

  e.g. low level instruction paravirtualization. It must be

       initialized very early before platform detection.

- virtualization support needs more functionality like binary patch.

  Probably the calling overhead might not be very large compared to the

  emulation overhead of virtualization. However in the native case, the

  overhead should be eliminated completely.

  A single kernel binary should run on each environment including native,

  and the overhead of paravirt_ops on native environment should be as

  small as possible.

- for full virtualization technology, e.g. KVM/IA64 or

  Xen/IA64 HVM domain, the result would be

  (the emulated platform machine vector. probably dig) + (pv_ops).

  This means that the virtualization support layer should be under

  the machine vector layer.

Possibly it might be better to move some function pointers from

paravirt_ops to machine vector. In fact, Xen domU case utilizes both

pv_ops and machine vector.

IA64 paravirt_ops

-----------------

In this section, the concrete paravirt_ops will be discussed.

Because of the architecture difference between ia64 and x86, the

resulting set of functions is very different from x86 pv_ops.

- C function pointer tables

They are not very performance critical so that simple C indirect

function call is acceptable. The following structures are defined at

this moment. For details see linux/include/asm-ia64/paravirt.h

  - struct pv_info

    This structure describes the execution environment.

  - struct pv_init_ops

    This structure describes the various initialization hooks.

  - struct pv_iosapic_ops

    This structure describes hooks to iosapic operations.

  - struct pv_irq_ops

    This structure describes hooks to irq related operations

  - struct pv_time_op

    This structure describes hooks to steal time accounting.

- a set of indirect calls which need optimization

Currently this class of functions correspond to a subset of IA64

intrinsics. At this moment the optimization with binary patch isn't

implemented yet.

struct pv_cpu_op is defined. For details see

linux/include/asm-ia64/paravirt_privop.h

Mostly they correspond to ia64 intrinsics 1-to-1.

Caveat: Now they are defined as C indirect function pointers, but in

order to support binary patch optimization, they will be changed

using GCC extended inline assembly code.

- a set of macros for hand written assembly code (.S files)

For maintenance purpose, the taken approach for .S files is single

source code and compile multiple times with different macros definitions.

Each pv_ops instance must define those macros to compile.

The important thing here is that sensitive, but non-privileged

instructions must be paravirtualized and that some privileged

instructions also need paravirtualization for reasonable performance.

Developers who modify .S files must be aware of that. At this moment

an easy checker is implemented to detect paravirtualization breakage.

But it doesn't cover all the cases.

Sometimes this set of macros is called pv_cpu_asm_op. But there is no

corresponding structure in the source code.

Those macros mostly 1:1 correspond to a subset of privileged

instructions. See linux/include/asm-ia64/native/inst.h.

And some functions written in assembly also need to be overrided so

that each pv_ops instance have to define some macros. Again see

linux/include/asm-ia64/native/inst.h.

Those structures must be initialized very early before start_kernel.

Probably initialized in head.S using multi entry point or some other trick.

For native case implementation see linux/arch/ia64/kernel/paravirt.c.