On Wed, Jun 06, 2007 at 02:12:22PM +0200, Bodo Eggert wrote:
> Change the description of CONFIG_*HIGHMEM* to reflect "lost" memory due to
> PCI space and the existence of the NX flag.
>
> Signed-Off-By: Bodo Eggert <[EMAIL PROTECTED]>
> ---
> I made this quick patch using the information from LKML as I remembered
> it. Please verify.
>
> --- 2.6.21/arch/i386/Kconfig.ori 2007-06-06 13:41:09.000000000 +0200
> +++ 2.6.21/arch/i386/Kconfig 2007-06-06 14:07:40.000000000 +0200
> @@ -495,8 +495,8 @@ config NOHIGHMEM
> bool "off"
> depends on !X86_NUMAQ
> ---help---
> - Linux can use up to 64 Gigabytes of physical memory on x86 systems.
> - However, the address space of 32-bit x86 processors is only 4
> + Linux can use up to 64 Gigabytes of physical address space on x86
> + systems. However, the address space of 32-bit x86 processors is only 4
> Gigabytes large. That means that, if you have a large amount of
> physical memory, not all of it can be "permanently mapped" by the
> kernel. The physical memory that's not permanently mapped is called
> @@ -510,8 +510,15 @@ config NOHIGHMEM
> by the kernel to permanently map as much physical memory as
> possible.
>
> - If the machine has between 1 and 4 Gigabytes physical RAM, then
> +
> + If the machine has between 1 and 3.5 Gigabytes physical RAM, then
> answer "4GB" here.
> +
> + The PCI address space will usurally take 512 MB or 1 GB of address
usually
> + space. This address space is unavailable to RAM, but depending on the
> + chipset (and BIOS settings), memory overlapping the PCI address space
> + may be mapped beyond the 4 GB limit and be available using "64GB".
> +
>
> If more than 4 Gigabytes is used then answer "64GB" here. This
> selection turns Intel PAE (Physical Address Extension) mode on.
> @@ -520,6 +527,10 @@ config NOHIGHMEM
> processors (Pentium Pro and better). NOTE: If you say "64GB" here,
> then the kernel will not boot on CPUs that don't support PAE!
>
> + An additional benefit of the 64GB-Mode is the availability of the
> + no-execute-pageflag, which can be used to prevent some attacks from
> + injecting malicious code into applications.
> +
> The actual amount of total physical memory will either be
> auto detected or can be forced by using a kernel command line option
> such as "mem=256M". (Try "man bootparam" or see the documentation of
> @@ -532,14 +543,14 @@ config HIGHMEM4G
> bool "4GB"
> depends on !X86_NUMAQ
> help
> - Select this if you have a 32-bit processor and between 1 and 4
> + Select this if you have a 32-bit processor and between 1 and 3.5
> gigabytes of physical RAM.
>
> config HIGHMEM64G
> - bool "64GB"
> + bool "64GB (enables no-execute memory protection if available)"
> depends on X86_CMPXCHG64
> help
> - Select this if you have a 32-bit processor and more than 4
> + Select this if you have a 32-bit processor and more than 3.5
> gigabytes of physical RAM.
>
> endchoice
Seems like an improvement to me. To fully explain how it could be 3 or
3.5 or 3.25 or who knows how many GB you can actually use without PAE
would probably require writing a small novel. Certainly talking about
address space instead of amounts of physical memory is more correct.
--
Len Sorensen
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