Re: [PATCH] Move assembly code out of the kernel

[chaac@nic.fi]

Bean [bean123ch@gmail.com] kirjoitti: 
> Hi,
> 
> Currently, most assembly code are in startup.S. This is normally used
> to ensure that the function address are below 1m, which is required if
> it would switch to real mode and call bios services. However, this
> make the kernel larger. For example, the biosdisk functions are only
> used by biodisk module, they should not be placed inside the kernel.
> 
> This patch support splitting such code from startup.S. For example, we
> create a new module biosdisk_stub.mod for assembly code of biosdisk.
> Instead of call prot_to_real and real_to_prot, we call
> grub_call_real_stub to enter real mode. grub_call_real_stub would copy
> the code to real mode and do the mode switch.
> 
> To avoid unnecessary memory transfer, grub_call_real_stub would not
> erase the real mode stub when it's done, so that it can be used
> directly next time. When the stub area is full, it zero it out and
> start anew. The area uses a simple verification method so that the old
> mapping is invalidated, the code would need be copied again on their
> next use.
> 
> The patch shows how to do it for the biosdisk module, here is the new
> grub_biosdisk_rw_int13_extensions function.
> 
> REAL_STUB_START(grub_biosdisk_rw_int13_extensions)
> 	movb	%dh, %ah
> 	movw	%cx, %ds
> 	int	$0x13		/* do the operation */
> 	movb	%ah, %dl	/* save return value */
> 	lret
> REAL_STUB_END(grub_biosdisk_rw_int13_extensions)
> 
> FUNCTION(grub_biosdisk_rw_int13_extensions)
> 	pushl	%ebp
> 	pushl	%esi
> 
> 	/* compute the address of disk_address_packet */
> 	movw	%cx, %si
> 	xorw	%cx, %cx
> 	shrl	$4, %ecx	/* save the segment to cx */
> 
> 	/* ah */
> 	movb	%al, %dh
> 
> 	leal	grub_biosdisk_rw_int13_extensions_stub, %eax
> 	call	EXT_C(grub_call_real_stub)
> 
> 	movb	%dl, %al	/* return value in %eax */
> 
> 	popl	%esi
> 	popl	%ebp
> 
> 	ret
> 
> Real mode code is enclosed between REAL_STUB_START and REAL_STUB_END,
> no need to use .code16 and .code32 as it's handled by the macro. In
> the main function, use
> 
> 	leal	grub_biosdisk_rw_int13_extensions_stub, %eax
> 	call	EXT_C(grub_call_real_stub)
> 
> to invoke grub_call_real_stub. grub_biosdisk_rw_int13_extensions_stub
> is defined in the REAL_STUB_START macro.
> 
> This same method can be applied to loaders, vbe, etc. In fact, almost
> all function behind grub_call_real_stub can be moved out of startup.S.

I have made generic function that does basically the same thing for bios service 0x10 (video). In that modification you prepare registers structure that will be configured during real mode switching. I am yet to commit it for review, but I think it would be more generic way to do this. When I come back from my holiday I will commit the code for review.

So please wait a bit before committing this :)

Thanks,
Vesa Jääskeläinen