Saveoops: Making Kexec purgatory position-independent?
Ahmed S. Darwish
darwish.07 at gmail.com
Sun Feb 27 08:24:09 EST 2011
On Sat, Feb 26, 2011 at 04:57:30PM -0800, Eric W. Biederman wrote:
> "H. Peter Anvin" <hpa at zytor.com> writes:
> > I can't see any sane reason to *not* make kexec purgatory
> > position-independent. It is the obvious thing to do.
> This isn't a case of the code not being position independent. This is
> case of where the relocations are applied.
> I can see a couple of handling this with different tradeoffs.
> 1) We teach bootloaders how to load two kernels at once. This
> completely avoids the purgatory, as it is replaced by code in the
> bootloader that already exists to load the primary kernel and setup
> it's arguments.
This is in fact my plan. Using Syslinux, I loaded 'purgatory.ro' to RAM
thinking that it will still be needed. Re-checking the purgatory code
now after reading above note, it seems it does 5 important points:
a) reset the VGA (if instructed)
b) reset the PIC to legacy mode (if instructed)
c) check the overall integrity of the second kernel image (SHA-2)
d) setup the environment for second kernel entry (switch back to
32-bit protected mode in x86-64, reset registers, etc)
e) saves the first 640K in a backup region
So (a) and (b) can be done elsewhere if needed; (c) isn't needed cause
if the bootloader corrupts images, we have bigger problems; (d) can be
done as a stub; (e), on the contrary of kdump, isn't critical for my
Am I missing an important detail?
> 2) We add minimal relocation processing to purgatory, allowing us to do
> the setup for the second kernel extremely early and allow it to be
> compiled into the first kernel.
> 3) We come up with a scheme where we don't share code and the first
> kernel copies the firmware information to place where the second
> kernel can get at it, and uses it's own home grown stub and not
Sorry, but how the third point differs from the first? I thought they
> I think this whole thing can be prototyped easily with a getting /sbin/kexec
> to load to a fixed address and then baking that section into the primary
> kernel. ...
I'll prototype this now by loading the second kernel (bzImage), using
syslinux, without the purgatory. Let's hope I won't face many
> ... I'm not convinced that directly using /sbin/kexec is the right
> way forward to handle the general case. This is something where the
> devil is in the details.
Lots of details per se; spent last week exploring Kexec user and
kernel code to understand how it does its magic.
More information about the kexec