[Xen-devel] Re: [PATCH] SeaBIOS/Xen: Compute the low RAM memory size in the BDA according to the e820

[Kevin O'Connor <kevin@xxxxxxxxxxxx>]

On Mon, Nov 14, 2011 at 02:03:38AM +0000, Julian Pidancet wrote:
> On Mon, Nov 14, 2011 at 1:11 AM, Kevin O'Connor <kevin@xxxxxxxxxxxx> wrote:
> >
> > That wont work reliably - SeaBIOS (and option roms) can grow the EBDA.
> > It's not realistic to put a whole in the middle of the first 640k.
> >
> 
> When you say "grow" do you mean grow downwards ? Because if not, the
> ACPI info structure lives in 9F000, so there shouldn't be any problem
> reporting the 0-9F000 range as the memory size in the BDA.

Yes - the EBDA grows down.  Both SeaBIOS itself can grow the EBDA (see
pmm.c:zonelow_expand) and option roms can grow the EBDA.

> > As above, this doesn't look right - SeaBIOS will still locate the EBDA
> > at 9fc00 and nothing will stop it from growing it over the 9f000 area.
> >
> 
> The EBDA is a standard bios structure, can't we safely assume that
> it's size will hardly change ? If not, would you suggest that this
> ACPI info structure should be placed at a different location ? The
> problem with this is that it has to be in a location with a fixed
> address, and preferably in a location which is accessible by any AML
> interpreter.

Why does it have to be at a fixed location?  What structure is
actually placed at this address?

The AML interpreter should be able to see all of ram, so that doesn't
seem like an issue.

-Kevin

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