Comments about implementing kprobes with Xen paravirtualization kernels

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Comments about implementing kprobes with Xen paravirtualization kernels

William Cohen
Talked with Stephen Tweedie about kprobes in Xen. Right now kprobes is
disable in the Xen configuration because it doesn't work. Appearently
there are things that build in Xen, but do not work. The current
approach in Xen copies large chunks of code of specific architectures
into Xen, then changes are made on the code. This leads to a maintence
headaches. Having to copy patches over to Xen code and then modify
them because of the additional Xen changes.  This is one of the things
that needs to changes before Xen is accepted in the mainline Linux
kernel.

There are patches being generated for the FC5 kernel by taking
differences between the linus-2.6 (kernel.org) and merge-2.6
(xensource) kernels. These patches do not aways work because of the
additional patches we have on FC5, e.g. VSDO builds in FC5 xen kernels
but do not boot.

Need a list of all the patches used to implement kprobes in the
kernel. For this the related xen arch files could be generated and
this could be used as a starting point. Otherwise it is going to be
painful manual process to create the equivalent kprobes patch for Xen.
However, once there is a patch, Stephen thought it was quite likely
that the xensource people would be willing to accept it.

One open question was whether Xen allows the text of the kernel to be
modified.

-Will
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Re: Comments about implementing kprobes with Xen paravirtualization kernels

Jim Keniston
On Thu, 2005-12-01 at 09:35, William Cohen wrote:
> Talked with Stephen Tweedie about kprobes in Xen...
> ...
> Need a list of all the patches used to implement kprobes in the
> kernel.

Patches against which kernel?

> For this the related xen arch files could be generated and
> this could be used as a starting point. Otherwise it is going to be
> painful manual process to create the equivalent kprobes patch for Xen.
> However, once there is a patch, Stephen thought it was quite likely
> that the xensource people would be willing to accept it.
>
> One open question was whether Xen allows the text of the kernel to be
> modified.

Dan Stekloff posed this question on the Xen IRC, and the answer was yes,
a guest kernel can modify its text.

>
> -Will
>

Thanks very much.
Jim

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Re: Comments about implementing kprobes with Xen paravirtualization kernels

William Cohen
Jim Keniston wrote:
> On Thu, 2005-12-01 at 09:35, William Cohen wrote:
>
>>Talked with Stephen Tweedie about kprobes in Xen...
>>...
>>Need a list of all the patches used to implement kprobes in the
>>kernel.
>
>
> Patches against which kernel?

This would be any kprobe related patch after the corresponding xen files
was cloned (and mutated) from the original kernel file.

As far as what redhat is currently using. Looking at
http://download.fedora.redhat.com/pub/fedora/linux/core/development/SRPMS/

Currently generates the xen kernels from:
kernel-xen-2.6.12-1.13_FC5.src.rpm

The base kernels are generated from:

kernel-2.6.14-1.1735_FC5.src.rpm

>
>
>>For this the related xen arch files could be generated and
>>this could be used as a starting point. Otherwise it is going to be
>>painful manual process to create the equivalent kprobes patch for Xen.
>>However, once there is a patch, Stephen thought it was quite likely
>>that the xensource people would be willing to accept it.
>>
>>One open question was whether Xen allows the text of the kernel to be
>>modified.
>
>
> Dan Stekloff posed this question on the Xen IRC, and the answer was yes,
> a guest kernel can modify its text.

Good, so that is not an issue.

-Will
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Re: Comments about implementing kprobes with Xen paravirtualization kernels

William Cohen
William Cohen wrote:

> Jim Keniston wrote:
>
>> On Thu, 2005-12-01 at 09:35, William Cohen wrote:
>>
>>> Talked with Stephen Tweedie about kprobes in Xen...
>>> ...
>>> Need a list of all the patches used to implement kprobes in the
>>> kernel.
>>
>>
>>
>> Patches against which kernel?
>
>
> This would be any kprobe related patch after the corresponding xen files
> was cloned (and mutated) from the original kernel file.
>
> As far as what redhat is currently using. Looking at
> http://download.fedora.redhat.com/pub/fedora/linux/core/development/SRPMS/
>
> Currently generates the xen kernels from:
> kernel-xen-2.6.12-1.13_FC5.src.rpm
>
> The base kernels are generated from:
>
> kernel-2.6.14-1.1735_FC5.src.rpm

I should add there is work to sync those up so kernel-xen uses patches
on the same version of the kernel being used for base kernel.

-Will