On 08/04/2010 09:38 AM, Gleb Natapov wrote:
But even if it wasn't it can potentially create havoc. I think we
currently believe that the northbridge likely never forwards RAM
access to a device so this doesn't fit how hardware would work.
Good point.
More importantly, BIOSes and ROMs do very funny things with RAM.
It's not unusual for a ROM to muck with the e820 map to allocate RAM
for itself which means there's always the chance that we're going to
walk over RAM being used for something else.
ROM does not muck with the e820. It uses PMM to allocate memory and the
memory it gets is marked as reserved in e820 map.
PMM allocations are only valid during the init function's execution.
It's intention is to enable the use of scratch memory to decompress or
otherwise modify the ROM to shrink its size.
If a ROM needs memory after the init function, it needs to use the
traditional tricks to allocate long term memory and the most popular one
is modifying the e820 tables.
See src/arch/i386/firmware/pcbios/e820mangler.S in gPXE.
Regards,
Anthony Liguori
--
Gleb.
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