Re: [RFC] ELF Relocatable x86 and x86_64 bzImages

[Eric W. Biederman]

Vivek Goyal <vgoyal@xxxxxxxxxx> writes:

> On Tue, Aug 01, 2006 at 04:58:49AM -0600, Eric W. Biederman wrote:
>> 
>> The problem:
>> 
>> We can't always run the kernel at 1MB or 2MB, and so people who need
>> different addresses must build multiple kernels.  The bzImage format
>> can't even represent loading a kernel at other than it's default address.
>> With kexec on panic now starting to be used by distros having a kernel
>> not running at the default load address is starting to become common.
>> 
> Hi Eric,
>
> There seems to be a small anomaly in the current set of patches for i386.
>
> For example if one compiles the kernel with CONFIG_RELOCATABLE=y
> and CONFIG_PHYSICAL_START=0x400000 (4MB) and he uses grub to load
> the kernel then kernel would run from 1MB location. I think user would
> expect it to run from 4MB location.

Agreed.  That is a non-intuitive, and should probably be fixed.

> I think distro's might want to keep above config options enabled. 
> CONFIG_RELOCATABLE=y so that kexec can load kdump kernel at a 
> different address and CONFIG_PHYSICAL_START=non 1MB location, to
> extract better performance. (As we had discussions on mailing list
> some time back.)
>
> In principle this is a limitation on boot-loaders part but as we can
> not fix the boot-loaders out there, probably we can try fixing it
> at kernel level.
>
> What I have done here is that decompressor code will determine the
> final resting place of the kernel based on boot loader type. So 
> if I have been loaded by kexec, I am supposed to run from loaded address
> otherwise I am supposed to run from CONFIG_PHYSICAL_START as I have been
> loaded at 1MB address due to boot loader limitation and that's not the
> intention.
>
> A prototype patch is attached with the mail. I have assumed that I can
> assign a boot loader type id 9 to kexec (Documentation/i386/boot.txt).
> Also assuming that all other boot loaders apart from kexec have got 1MB
> limitation. If not, its trivial to include their boot loader ids also.
>
> I have tested this patch and it works fine. What do you think about
> this approach ?

I think there is some value in it. But I need to digest it.

I have a cold right now and am running pretty weak, so it is going to take me
a little bit to look at this.

I don't like taking action based upon bootloader type.  As that assumes
all kinds of things.  But having better rules for when we perform relocation
makes sense.  There might be a way to detect coming from setup.S

I gave it some care last time, I worked through this and it didn't quite work.

I guess the practical question is do people see a real performance benefit
when loading the kernel at 4MB?

Possibly the right solution is to do like I did on x86_64 and simply remove
CONFIG_PHYSICAL_START, and always place the kernel at 4MB, or something like
that.

The practical question is what to do to keep the complexity from spinning
out of control.  Removing CONFIG_PHYSICAL_START would seriously help with
that.

Eric
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