Re: [Xen-devel] X86_64 and 4GB RAM using Flat Memory Model?

["John Hannfield" <hal9020@xxxxxxxxx>]

Hi Kier,

> So the issue here is that grub has for some reason been upset by the values
> returned to it by the e820 bios command, and has decided to fall back to a
> simpler bios command which only tells you amount of memory up to the first
> memory hole. This wouldn't necessarily affect Linux because it interrogates
> the bios itself, using different code which isn't affected by this bug
> (either it's a bios bug which Linux explicitly works around or is simply not
> susceptible to; or it's a grub bug which Linux doesn't have).
OK, I understand now.
Thanks for the explanation.

>  1. Dig into why GRUB is bailing on the e820 map, and come up with a grub
> patch to fix (or work around) the bug. This will require modifying,
> building, installing grub, rebooting the machine, etc etc. It's a pain in
> the arse.
OK, I will look in to that. I have tested the latest debian grub 0.97-27
with some patches which fix BIOS e820 fixes people recommended, but
still the same.

>  2. Boot via a different method (basically that would mean pxe). This might
> be convenient or impossible, depending on your machine's local network
> environment.
Hmmm. Maybe.

>  3. There is no command-line option for overrding the e820 map. We could
> lash one up, or I can give you a basic patch for you to fill in the blanks
> with your memory map. You can then build special Xen with hardcoded map for
> that box.
That would be great. I have the memory map from the vanilla linux boot, so
if you can show me where to hardcode that in, I will give it a shot. I always
compile xen from source anyway for each server.

> I should add the slightly longer term (post 3.0.5) solution, which will be
> that Xen can return to real mode and interrogate e820 itself. We could at
> least provide this as a command-line option, even if it's not the default.
Ok, that would be useful.

Thanks

John

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