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Re: [Xen-devel] [PATCH 0/2][PV-on-HVM] Enable Front-end drivers for 2.4

To: ddutile@xxxxxxxxxx
Subject: Re: [Xen-devel] [PATCH 0/2][PV-on-HVM] Enable Front-end drivers for 2.4 kernels
From: Ben Guthro <bguthro@xxxxxxxxxxxxxxx>
Date: Thu, 07 Feb 2008 09:53:02 -0500
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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Hi Don,

I thought I'd see if these patches might be available - or if they are still in development?


Don Dutile wrote:

Our apologies for the late notice, but RedHat has a set of
patches that apply to a xen-3.1.0 tarball to yield a set
of functioning pv-on-hvm drivers for RHEL3-U8 (& U9).
[A reduced set of patches would yield the same result w/3.1.2 .]

Our plans are to provide those patches to xen-devel in January,
after further cleanup (so they don't break 2.6 builds).

The patches have some similarities & differences to those listed
below.  More specifically, they have what is described in 4, 5, & 6
(although I haven't had the time to compare the two implementations, but
a wheel typically comes out round ;-) ).
The patches differ in that h-file diffs are handled in the compat-include
directory. Additionally, the block device support is handled by a new set of
interfaces, reducing potential issues of modifying this code for
Linux 2.4 breaking Linux 2.6, or vice versa.
The patches don't include a build system as described below, which
sounds like a clean way to handle those diffs, and avoid 2.4 vs 2.6 changes/diffs
over time (much like our separate block interface support split for 2.4
avoids conflicts/issues w/2.6).

The heavy lifting for this support was done by Herbert Xu.
I've been adding bug fixes and getting it into a clean set of patches that can create an rpm that can be added to a rhel3 kernel, as well as working with a number of testers asking for this support on rhel3.

We'll look at the following patches in detail after the New Year.
(We turn into a pumpkin next week, and the fairy tale ends 1/2/08),
and see what we can meld into a patch set that takes the best
of both worlds.

- Don

Ben Guthro wrote:
This patch enables front end drivers to build under Linux 2.4.
Specifically, the 2.4.21-47 kernel is used. This corresponds to RedHat Linux 3 update 8

Changes were made in two areas. Files were changed in the unmodified tree as well as the sparse tree. The latter corresponds to the drivers/xen tree in the Linux 2.6.18 kernel and will be referred to as the "Linux driver tree" in
the remainder of this note.

In the unmodified tree, changes were related to build system modifications, addition of missing header files, implementation of the generic device model code for kernel 2.4 and all other nuggets required to compile front end drivers
under kernel 2.4.

In the Linux driver tree, changes made were located almost entirely in the front end drivers area. Most of these were related to implementation of compatibility macros and replacement of APIs which evolved, were added or removed between kernels 2.4 and 2.6. Where a one to one replacement of a specific call was not possible, blocks of code surrounded by kernel version specific preprocessor directives were added. One instance of this is disk geometry processing.

Below is a more detailed list of changes made in the unmodified tree.

1. Build system.  For each Kbuild file in the front driver area, a
corresponding K24build file has been created. There, 2.4 style targets are used. The main Makefile for each driver references appropriate "K" file
  depending on the kernel version the driver is being built for.
2. Nonexistent header files. Header files included in front end drivers which do not exist under kernel 2.4 were replaced by dummy headers. These, in turn, include compatibility headers to further resolve differences between kernel 2.4 and 2.6. Dummy header files reside in the compat-include/linux
3. Block interface. Changed APIs are handled through compatibility macros whose
  names are usually of the form compat_<original function name>().  This
  applies to:
a. end request processing; Note that some of these macros take the same number of arguments as original 2.6 APIs. The change of name is necessary because, while the corresponding 2.4 API exists, the number or type of
     arguments might have changed.  This is the case for
end_that_request_first(), for example. Additionally, as also happens to be the case with this particular API, the way in which some APIs are called varies between kernels 2.4 and 2.6. Specifically, under kernel 2.6, end_that_request_first() is called once with a pointer to the request being currently processed. The rest is done by the kernel. However, under kernel 2.4, this API is called repeatedly until a certain return code is obtained (which signals that the kernel is done with the current
     request).  This difference of having to call it once (2.6) or,
     potentially, many times (2.4) is covered in the corresponding
     compatibility macro.
  b. geometry calculations
  c. references to bio and bio_vec structures are now translated into
     references to buffer_head structures
d. resolution of driver's private data area pointer (struct blkfront_info
  e. resolution of the generic disk pointer
4. Work queue interface. This is now implemented using scheduler task queue. 5. Kernel thread interface. Those interfaces which are not defined under kernel 2.4 are implemented in the compatibility header file using 2.4 versions of
  thread functions.
6. Generic device model. A simplified version of device model interfaces was implemented to allow front end drivers to compile under kernel 2.4. All
  required structures appear in the compatibility header file.  All 2.4
versions of device model interfaces are implemented in platform-compat.c in
  platform-pci.o driver.

This list details changes made in the Linux driver tree.

1. Generic kernel compatibility header file.  Instead of including
xen/platform-compat.h which is compiled in only conditionally, a generic compatibility header is included. This file, named kerncompat.h is included unconditionally and contains all compatibility macros used in front end drivers. Moreover, kerncompat.h conditionally includes platform-compat.h just as it was done in the original front end driver code. Unconditional
  usage of kerncompat.h is necessary to give front end drivers access to
  compatibility macros.
2. Disk driver initialization and setup. Blocks of code needed to handle generic disk operation were added and are compiled for kernels below 2.6.0. 3. Partition processing. Blocks of code needed to process partition table updates and geometry inquires were added. These are conditionally compiled
  for kernels below 2.6.0 only.

Signed-off-by: Paul Burkacki <pburkacki@xxxxxxxxxxxxxxx>
Signed-off-by: Ben Guthro <bguthro@xxxxxxxxxxxxxxx>

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