linux-kernel - Re: [PATCH 17/20] x86_64: Remove CONFIG_PHYSICAL

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Date:	Mon, 20 Nov 2006 19:02:42 +0900
From:	"Magnus Damm" <magnus.damm@...il.com>
To:	vgoyal@...ibm.com
Cc:	"linux kernel mailing list" <linux-kernel@...r.kernel.org>,
	"Reloc Kernel List" <fastboot@...ts.osdl.org>,
	ebiederm@...ssion.com, akpm@...l.org, ak@...e.de, hpa@...or.com,
	lwang@...hat.com, dzickus@...hat.com, pavel@...e.cz, rjw@...k.pl
Subject: Re: [PATCH 17/20] x86_64: Remove CONFIG_PHYSICAL_START

On 11/18/06, Vivek Goyal <vgoyal@...ibm.com> wrote:
> On Sat, Nov 18, 2006 at 10:14:31AM +0900, Magnus Damm wrote:
> > On 11/18/06, Vivek Goyal <vgoyal@...ibm.com> wrote:
> > >I am about to add relocatable kernel support which has essentially
> > >no cost so there is no point in retaining CONFIG_PHYSICAL_START
> > >and retaining CONFIG_PHYSICAL_START makes implementation of and
> > >testing of a relocatable kernel more difficult.
> > >
> > >Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
> > >Signed-off-by: Vivek Goyal <vgoyal@...ibm.com>
> > >---

[snip]
 >linux-2.6.19-rc6-reloc/arch/x86_64/mm/fault.c~x86_64-Remove-CONFIG_PHYSICAL_START
  2006-11-17 00:12:50.000000000 -0500
> > >+++ linux-2.6.19-rc6-reloc-root/arch/x86_64/mm/fault.c  2006-11-17
> > >00:12:50.000000000 -0500
> > >@@ -644,9 +644,9 @@ void vmalloc_sync_all(void)
> > >                        start = address + PGDIR_SIZE;
> > >        }
> > >        /* Check that there is no need to do the same for the modules
> > >        area. */
> > >-       BUILD_BUG_ON(!(MODULES_VADDR > __START_KERNEL));
> > >+       BUILD_BUG_ON(!(MODULES_VADDR > __START_KERNEL_map));
> > >        BUILD_BUG_ON(!(((MODULES_END - 1) & PGDIR_MASK) ==
> > >-                               (__START_KERNEL & PGDIR_MASK)));
> > >+                               (__START_KERNEL_map & PGDIR_MASK)));
> > > }
> >
> > This code looks either like a bugfix or a bug. If it's a fix then
> > maybe it should be broken out and submitted separately for the
> > rc-kernels?
> >
>
> Magnus, Eric got rid of __START_KERNEL because he was compiling kernel
> for physical addr zero which made __START_KERNEL and __START_KERNEL_map
> same, hence he got rid of __START_KERNEL. That's why above change.
>
> But compiling for physical address zero has got drawback that one can
> not directly load a vmlinux as it shall have to be loaded at physical
> addr zero. Hence I changed the behavior back to compile the kernel for
> physical addr 2MB. So now __START_KERNEL = __START_KERNEL_map + 2MB.
>
> Now it makes sense to retain __START_KERNEL. I have done the changes.

I misunderstood and thought __START_KERNEL was a physical address and
__START_KERNEL_map was a virtual one. But now I understand. Thank you.

>
> > >diff -puN include/asm-x86_64/page.h~x86_64-Remove-CONFIG_PHYSICAL_START
> > >include/asm-x86_64/page.h
> > >---
> > >linux-2.6.19-rc6-reloc/include/asm-x86_64/page.h~x86_64-Remove-CONFIG_PHYSICAL_START        2006-11-17 00:12:50.000000000 -0500
> > >+++ linux-2.6.19-rc6-reloc-root/include/asm-x86_64/page.h       2006-11-17
> > >00:12:50.000000000 -0500
> > >@@ -75,8 +75,6 @@ typedef struct { unsigned long pgprot; }
> > >
> > > #endif /* !__ASSEMBLY__ */
> > >
> > >-#define __PHYSICAL_START       _AC(CONFIG_PHYSICAL_START,UL)
> > >-#define __START_KERNEL         (__START_KERNEL_map + __PHYSICAL_START)
> > > #define __START_KERNEL_map     _AC(0xffffffff80000000,UL)
> > > #define __PAGE_OFFSET           _AC(0xffff810000000000,UL)
> >
> > I understand that you want to remove the Kconfig option
> > CONFIG_PHYSICAL_START and that is fine with me. I don't however like
> > the idea of replacing __PHYSICAL_START and __START_KERNEL with
> > hardcoded values. Is there any special reason behind this?
> >
>
> All the hardcodings for 2MB have disappeared in final version. See next
> patch in the series which actually implements relocatable kernel. Actually
> the whole logic itself has changed hence we did not require these
> hardcodings. This patch retains these hardcodings so that even if somebody
> removes the top patch, kernel can be compiled and booted.
>
> So bottom line, all the hardcodings are not present once all the patches
> have been applied.

My gut feeling said a big no when I saw that you replaced constants
with hardcoded values. But if the hardcoded values disappear when all
patches are applied then I'm happy!

> > The code in page.h already has constants for __START_KERNEL_map and
> > __PAGE_OFFSET (thank god) and none of them are adjustable via Kconfig.
> > Why not change as little as possible and keep __PHYSICAL_START and
> > __START_KERNEL in page.h and the places that use them but remove
> > references to CONFIG_PHYSICAL_START in Kconfig, defconfig, and page.h?
>
> Good suggestion. Now I have retained __START_KERNEL. But did not feel
> the need to retain __PHYSICAL_START. It will be used only at one place
> in page.h

Just to nitpick, isn't the 2M value used both in page.h and head.S? I
don't fully understand the reason why you are hardcoding, but it is
not that important. I think this version of the patch is much better.
Thank you!

/ magnus
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