Re: [PATCH] Fix alignment of early reservation for EBDA

Alexander van Heukelum
2008-02-25T16:54:50Z

On Sun, 24 Feb 2008 18:18:16 -0800, "H. Peter Anvin" <hpa@zytor.com>
said:
> Alexander van Heukelum wrote:
> > early res: 3 [9f000-9ffff] EBDA
> >
> > Is it really necessary to force the allocation to a page boundary?
>
> It is, but that rounding gets done in reserve_bootmem() anyway, so there
> is no need for the arch-specific code to do it.
>
> The 32-bit EBDA code hard-codes a size of 4K, which is probably equally
> wrong; my gut feel is that the right thing to do is to reserve from the
> EBDA up to the 640K mark (some BIOSes use an area like that for SMM
> stuff), possibly with some sanity checking.

Then, how about reserving everything from the end of conventional memory
up to the 1Mb mark? Like this:

/*
* The BIOS places the EBDA/XBDA at the top of conventional
* memory, and usually decreases the reported amount of
* conventional memory (int 0x12) too.
*/
static __init void reserve_ebda(void)
{
unsigned int lowmem, ebda_addr;

/* end of low (conventional) memory */
lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
lowmem <<= 10;

/* start of EBDA area */
ebda_addr = *(unsigned short *)__va(BIOS_EBDA_SEGMENT);
ebda_addr <<= 4;

/* Fixup: bios puts an EBDA in the top 64K segment */
/* of conventional memory, but does not adjust lowmem. */
if ((lowmem - ebda_addr) <= 0x10000)
lowmem = ebda_addr;

/* Fixup: bios does not report an EBDA at all. */
/* Some old Dells seem to need 4k anyhow (bugzilla 2990) */
if ((ebda_addr == 0) && (lowmem >= 0x9f000))
lowmem = 0x9f000;

/* Paranoia: should never happen, but... */
if (lowmem >= 0x100000)
lowmem = 0xa0000;

/* reserve all memory between lowmem and the 1MB mark */
reserve_early(lowmem, 0x100000, "BIOS reserved");
}

Greetings,
Alexander
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Alexander van Heukelum
heukelum@fastmail.fm

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