Re: COMMAND_LINE_SIZE increasing in 2.6.11-rc1-bk6

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

Matt Domsch wrote:
> On Fri, Jan 21, 2005 at 08:11:44AM +0100, Andi Kleen wrote:
> 
>>>I really suggest to push this limit to 4k. My reason is that under UML I 
>>>need to put a lot of stuff in command line and uml crash if I not extend 
>>>this limit. Can we make it depend on arhitecture?
>>
>>It's dependent on the architecture already. I would like to enable
>>it on i386/x86-64 because the kernel command line is often used
>>to pass parameters to installers, and having a small limit there
>>can be awkward.
>>
>>But first need to figure out what went wrong with EDD. 
>>
>>Matt D., do you have thoughts on this?
> 
> 
> It is definitely boot-loader dependent.  Simply changing
> COMMAND_LINE_SIZE from 256 to 2048 in the kernel isn't enough.
> 
> There are 2 ways the command line is passed from the boot loader into
> the kernel.
> 
> Boot loader version <= 0x0201  (which LILO uses)
> I believe the command line is located at the end of what was known as
> the 'empty zero page', now known as the boot parameters.  This part is
> black magic to me.
> 
> Boot loader version >= 0x0202  (which GRUB uses)
> command line can be essentially any size, located anywhere in memory,
> and the boot loader tells the kernel where to find it.  The EDD real
> mode code uses only this case for parsing the command line, and if an
> older loader is used, EDD skips parsing the command line looking
> for its options.
> 
> 
> There's little space left in the boot parameters block, my EDD code
> uses nearly all that was remaining, and could use some more if it were
> available.  Having a longer command line would be nice too.  I spoke
> with hpa at OLS last summer about this, and he offered to help.
> Peter?
> 

The protocol itself doesn't encode it, but before we extend it for 
protocol >= 0x0202 we need to make sure that older kernels don't break 
if they get a very long command line (truncation is OK, crashing is 
not.)  If they do crash, we need to add a field in the header.

I don't see any reason why the boot parameter block can't be more than 
one page long.  I think today that it's just a static structure.

	-hpa