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

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 14/11/2011 03:36, "Kevin O'Connor" <kevin@xxxxxxxxxxxx> wrote:

>> Like I said, I'm not an ACPI expert. But let say we decide to move
>> this ACPI info structure to some other area, where there's less risk
>> for it to be overwritten, like somewhere above 0xFC000000, wouldn't
>> that prevent some Operating System with limited memory capabilities to
>> access it ?
> 
> If the OS can handle AML it can handle 32bit addresses.
> 
>> Besides, it seems that SeaBIOS manages itself the space between
>> 0xFC000000 and 4G, so it seems difficult to imagine to have a reserved
>> space with a fixed address in there.
> 
> On Xen, the PCI init code isn't used, so assuming this struct doesn't
> need to live in real "ram", I think it could live just about anywhere
> past the end of ram.  Even with pciinit.c, addresses over 0xfc00000
> (with the exception of a few bytes for hpet, apic, ioapic, and bios
> image) could be used.

I suggest we stick it at FC000000, and shift hvmloader's mem_alloc()
starting address up by one page to FC001000. The acpi build code will have
to manually mem_hole_populate_ram() that one page before writing to it. This
can then be documented in hvmloader/config.h which contains a description
of, and defines for, the system memory map. This is by far the easiest
solution to this problem; manually crafting an SSDT is a right pain in the
arse, whereas this is maybe a 5-line patch.

 -- Keir

> -Kevin
> 
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