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

["H. Peter Anvin" <h.peter.anvin@intel.com>]

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?  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.

> 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.

> 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.

	-hpa