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

[Ben Guthro <bguthro@virtualiron.com>]

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We tried to minimize the "ifdef mess" as much as possible to be 
encapulated in as few places as possible.
Which parts in particular concern you? If you could propose some 
suggestions - we could try another rev.

How would this patch lead to code divergence as submitted?

An ideal solution would be a 2.4.*-xen.hg tree which would support both 
PV, and PV-on-HVM solutions. However, that was not the goal of these 
patches.



Keir Fraser wrote:
> That's a lot of ifdef mess for a feature that most people probably don't
> want. I'm not sure what the right answer is for those who *do* want it. A
> driver kit for Linux 2.4 would be neat, but could just lead to code
> divergence.
>
>  -- Keir
>
>
> On 19/12/07 01:07, "Ben Guthro" <bguthro@virtualiron.com> 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
>> release.
>>
>> 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
>>    tree.
>> 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
>>       pointer)
>>    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@virtualiron.com>
>> Signed-off-by: Ben Guthro <bguthro@virtualiron.com>
>>
>>
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>> Xen-devel mailing list
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>> http://lists.xensource.com/xen-devel
>>     
>
>
>   


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