x86: default CONFIG_PHYSICAL_START and CONFIG_PHYSICAL_ALIGN to 16 MB

Default CONFIG_PHYSICAL_START and CONFIG_PHYSICAL_ALIGN each to 16 MB,
so that both non-relocatable and relocatable kernels are loaded at
16 MB by a non-relocating bootloader.  This is somewhat hacky, but it
appears to be the only way to do this that does not break some some
set of existing bootloaders.

We want to avoid the bottom 16 MB because of large page breakup,
memory holes, and ZONE_DMA.  Embedded systems may need to reduce this,
or update their bootloaders to be aware of the new min_alignment field.

[ Impact: performance improvement, avoids problems on some systems ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5aee453..50fbb47 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1455,9 +1455,7 @@ config KEXEC_JUMP
 
 config PHYSICAL_START
        hex "Physical address where the kernel is loaded" if (EMBEDDED || CRASH_DUMP)
-       default "0x1000000" if X86_NUMAQ
-       default "0x200000" if X86_64
-       default "0x100000"
+       default "0x1000000"
        ---help---
          This gives the physical address where the kernel is loaded.
 
@@ -1476,15 +1474,15 @@ config PHYSICAL_START
          to be specifically compiled to run from a specific memory area
          (normally a reserved region) and this option comes handy.
 
-         So if you are using bzImage for capturing the crash dump, leave
-         the value here unchanged to 0x100000 and set CONFIG_RELOCATABLE=y.
-         Otherwise if you plan to use vmlinux for capturing the crash dump
-         change this value to start of the reserved region (Typically 16MB
-         0x1000000). In other words, it can be set based on the "X" value as
-         specified in the "crashkernel=YM@XM" command line boot parameter
-         passed to the panic-ed kernel. Typically this parameter is set as
-         crashkernel=64M@16M. Please take a look at
-         Documentation/kdump/kdump.txt for more details about crash dumps.
+         So if you are using bzImage for capturing the crash dump,
+         leave the value here unchanged to 0x1000000 and set
+         CONFIG_RELOCATABLE=y.  Otherwise if you plan to use vmlinux
+         for capturing the crash dump change this value to start of
+         the reserved region.  In other words, it can be set based on
+         the "X" value as specified in the "crashkernel=YM@XM"
+         command line boot parameter passed to the panic-ed
+         kernel. Please take a look at Documentation/kdump/kdump.txt
+         for more details about crash dumps.
 
          Usage of bzImage for capturing the crash dump is recommended as
          one does not have to build two kernels. Same kernel can be used
@@ -1521,9 +1519,8 @@ config X86_NEED_RELOCS
 config PHYSICAL_ALIGN
        hex
        prompt "Alignment value to which kernel should be aligned" if X86_32
-       default "0x100000" if X86_32
-       default "0x200000" if X86_64
-       range 0x2000 0x400000
+       default "0x1000000"
+       range 0x2000 0x1000000
        ---help---
          This value puts the alignment restrictions on physical address
          where kernel is loaded and run from. Kernel is compiled for an