Re: [PATCH 00/14] RFC: x86: relocatable kernel changes

[Eric W. Biederman]

"H. Peter Anvin" <h.peter.anvin@xxxxxxxxx> writes:

> Eric W. Biederman wrote:
>> Peter do you plan to update pxelinux or other bootloaders to use the
>> relocatable kernel feature?
>
> Yes.
>
>> The direction of this patch seems reasonable.  The details are broken.
>> The common case for relocatable kernels today is kdump.  A situation
>> with very minimal memory.  In that situation the kernel needs to run
>> where we put it, modifying the kernel to not run where it gets put
>> is a problem.
>
> I thought in the kdump case you typically loaded it pretty high?  Either
> which way, kdump is always loaded by kexec, so it should just be a
> matter of updating kexec to zero the runtime_start field, no?

Yes.  In practice it doesn't matter. I just don't want to get into a
contest with the kernel about who knows better how to put the kernel
in memory the bootloader or the kernel decompressor.

> Basically
> this is the bootloader saying "do what I say, dammit."  Since the
> existing protocol doesn't have a way to unambiguously communicate one
> direction versus another (see below), it seems like a relatively small
> issue involving only one tool.  Suboptimal, yes.

The existing protocol doesn't have the option of anything else.

Physical start has always been <= the alignment for x86 and x86_64,
in any real world configuration.

Something goofy may have happened during unification, I thought I had
removed physical start as totally unnecessary from x86_64.

Hmmm....

In the non-kdump case this is interesting.  I know of instances where
kexec is burned in firmware.  So I am strongly reluctant to make anything
that feels like a true backwards incompatible change.

Those systems also don't have the stupid 15MB hole either.

>> With the code as it is today you can get the exact same behavior
>> by simply bumping up the minimum alignment to 16MB, and a lot less code
>> and no changes needed to any bootloaders.
>> 
>> Is your goal to setup a scenario where on small memory systems a bootloader
>> like pxelinux can support a relocatable kernel and load it a lower
>> address?  If so that seems reasonable.
>
> Yes.
>
>> With that said how about we change the logic to:
>> 
>> if (load_addr == legacy_load_addr) /* 0x100000 */
>> 	use config_physical_start
>> else if aligned
>> 	noop
>> else
>>         /* Crap this is bad, align the kernel and hope something works. */
>> 
>> That gets the desired behavior we override bootloaders that are not
>> smart and taking relocation into account.  I am really not comfortable
>> with having code that will override a bootloader doing something
>> reasonable.
>
> I'm not sure that is quite right either, because if alignment is
> configured to be 1 MB or less then 1 MB is a perfectly legitimate
> address for a relocating bootloader to want to use, even if it is not
> configured in.  It would be more than a bit odd to not have that be
> permitted.

On the 64bit kernel 2MB really is required.  We run at a fixed virtual
address and use 2MB pages. So anything less that 2MB really won't work.

So I think it would be a bad idea if we had bootloaders ignoring the
alignment.

With the suggested start address, it probably make sense to only
export our true alignment requirement.

>> I expect we will still want to update kexec to be able to take
>> advantage of loadtime_size (runtime_size seems like the wrong name).
>
> Well, it is the amount of memory the kernel needs during runtime (as
> opposed to during loading.)  I admit it's not an ideal name, though.  On
> the other hand, simply calling it kernel_start and kernel_size seemed
> ambiguous.

It is the amount of memory we need before a true memory allocator is
initialized.  Essentially text+data+bss.  How about we call it init_size?

Perhaps we should have:
init_size
best start (As a 64bit field please)
optimum align  (Or we flip it around)

Eric
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