linux-kernel - Re: [PATCH 02/14] x86, boot: honor CONFIG_PHYSICAL

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Date:	Fri, 8 May 2009 09:34:36 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, vgoyal@...hat.com, hbabu@...ibm.com,
	kexec@...ts.infradead.org, ying.huang@...el.com, mingo@...e.hu,
	tglx@...utronix.de, ebiederm@...ssion.com,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 02/14] x86, boot: honor CONFIG_PHYSICAL_START when relocatable

On Thu, May 07, 2009 at 03:26:50PM -0700, H. Peter Anvin wrote:
> From: H. Peter Anvin <hpa@...or.com>
> 
> Currently, when building a relocatable kernel, CONFIG_PHYSICAL_START
> is ignored.  This is undesirable, as we would like to keep the kernel
> out of ZONE_DMA and away from the possible memory hole at 15 MB (which
> some vendors for some bizarre reason still have.)
> 
> With this patch, CONFIG_PHYSICAL_START is considered the *minimum*
> address at which the kernel can be located; a relocating bootloader
> can locate it higher, but not lower.  This also restores the
> originally intended behavior that CONFIG_RELOCATABLE is functionally a
> noop if used with a non-relocating bootloader.
> 
> This patch also change movsb and stosb to movsl, stosl and stosq, to
> shave a small fraction off the boot time.

On the coding style side of thing...
> +     movl %ebp, %ebx
> +     cmpl %ebx, %eax
> +     jbe 1f
> +     movl %eax, %ebx

This looks messy with different idention of the operand.

Compare it to this:
> +	movl	%ebp, %ebx
> +	cmpl	%ebx, %eax
> +	jbe	1f
> +	movl	%eax, %ebx


I like the latter more with vertical alignment of the operand.
I see that you add spaces around operators (,) - good.



>   */
>  
>  #ifdef CONFIG_RELOCATABLE
> -	movl 	%ebp, %ebx
> -	addl    $(CONFIG_PHYSICAL_ALIGN - 1), %ebx
> -	andl    $(~(CONFIG_PHYSICAL_ALIGN - 1)), %ebx
> +	movl $LOAD_PHYSICAL_ADDR, %eax
> +	movl %ebp, %ebx
> +	cmpl %ebx, %eax
> +	jbe 1f
> +	movl %eax, %ebx
> +1:
> +	addl $(CONFIG_PHYSICAL_ALIGN - 1), %ebx
> +	andl $(~(CONFIG_PHYSICAL_ALIGN - 1)), %ebx
>  #else
>  	movl $LOAD_PHYSICAL_ADDR, %ebx
>  #endif
> +	movl %ebx, %edi		/* Save kernel target address */
>  
>  	/* Replace the compressed data size with the uncompressed size */
>  	subl input_len(%ebp), %ebx
> @@ -84,27 +90,30 @@ ENTRY(startup_32)
>  	addl $4095, %ebx
>  	andl $~4095, %ebx

Not part of your patch...
But this is most likely (PAGE_SIZE - 1) so it would be better to write so.


>  
> -/* Copy the compressed kernel to the end of our buffer
> +/*
> + * Set up the stack
> + */
> +	leal boot_stack_end(%ebx), %esp
> +	pushl %edi		/* Saved kernel target address */
> +
> +/*
> + * Copy the compressed kernel to the end of our buffer
>   * where decompression in place becomes safe.
>   */
>  	pushl %esi
> -	leal _ebss(%ebp), %esi
> -	leal _ebss(%ebx), %edi
> -	movl $(_ebss - startup_32), %ecx
> +	leal (_bss-4)(%ebp), %esi
> +	leal (_bss-4)(%ebx), %edi
> +	movl $(_bss - startup_32), %ecx
> +	shrl $2, %ecx

I do not see why you mess around with _bss here?
Do you use .bss for decompression?


Note: I do not speak x86 assembler so I have not tried to
      understand the functionality.

      I tired to grasp the changes around clear bss - but faild :-(

	Sam
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