Re: Is the x86-64 kernel size limit real?

["H. Peter Anvin" <hpa@zytor.com>]

Olivier Galibert wrote:
> On Thu, Jun 22, 2006 at 02:38:02PM -0700, H. Peter Anvin wrote:
>> It turns out x86-64, unlike i386, does still have a hardcoded limit,
>> but the limit in build.c is wrong:
>>
>> kernel/head.S:
>>         /* 40MB kernel mapping. The kernel code cannot be bigger than that.
>>            When you change this change KERNEL_TEXT_SIZE in page.h too. */
>>         /* (2^48-(2*1024*1024*1024)-((2^39)*511)-((2^30)*510)) = 0 */
>>
>> So this should be replaced by KERNEL_TEXT_SIZE in page.h, or better,
>> this should be done dynamically in x86-64 too.
> 
> Interesting.  KERNEL_TEXT_SIZE wouldn't work though, since that's the
> decompressed size while the 4Mb limit is on the compressed size.  As a
> datapoint, though, the uncompressed image is 15.7Mb, for a 4.5Mb
> compressed image.
> 

Oh, right.  In fact, the 4 MB "limit" for i386 was actually an 8 MB uncompressed limit, 
with a 2:1 ratio assumed... not very accurate.

The limit should be removed from the boot tools; since we're talking uncompressed limits 
those should be tested in the linker script if anywhere.

	-hpa