Re: [PATCH] reserve end-of-conventional-memory to 1MB on 32-bit

Ian Campbell
2008-02-28T21:10:42Z

On Thu, 2008-02-28 at 14:13 +0100, Alexander van Heukelum wrote:
> The 32-bit code still uses reserve_bootmem, so this is not really
> a unification with the 64-bit version of the ebda reservation code,
> but at least it provides the same detection logic and reserves the
> same areas.
>
> This does not crash immediately on qemu. No further testing was
> done! Otherwise:

I haven't tested extensively either but it does seem to solve the
problem for Xen.

Thanks!
Ian

>
> Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
>
> ---
>
> Hello Ian, Ingo,
>
> This patch should do the same reservations as the reserve_early
> patch for 64 bit. Maybe you could try if this solves the problems
> you are seeing with Xen?
>
> On the other hand, Xen should be able to use those legacy ranges
> as normal memory. Why doesn't it work? Does Xen use a special
> bootloader? In that case it could just add the reservation to
> the e820 memory map that is provided.
>
> Another possible approach to the whole problem is just to ammend
> the e820 memory map in the boot-code. Thanks to hpa, it is now easy
> to change this code. Then the kernel can just trust the e820 memory
> map, and only 'sophisticated' bootloaders need to do something
> similar by themselves.
>
> Greetings,
> Alexander
>
> diff --git a/arch/x86/kernel/setup_32.c b/arch/x86/kernel/setup_32.c
> index a1d7071..e12cc31 100644
> --- a/arch/x86/kernel/setup_32.c
> +++ b/arch/x86/kernel/setup_32.c
> @@ -385,15 +385,47 @@ unsigned long __init find_max_low_pfn(void)
> return max_low_pfn;
> }
>
> +#define BIOS_EBDA_SEGMENT 0x40E
> +#define BIOS_LOWMEM_KILOBYTES 0x413
> +
> /*
> - * workaround for Dell systems that neglect to reserve EBDA
> + * The BIOS places the EBDA/XBDA at the top of conventional
> + * memory, and usually decreases the reported amount of
> + * conventional memory (int 0x12) too. This also contains a
> + * workaround for Dell systems that neglect to reserve EBDA.
> + * The same workaround also avoids a problem with the AMD768MPX
> + * chipset: reserve a page before VGA to prevent PCI prefetch
> + * into it (errata #56). Usually the page is reserved anyways,
> + * unless you have no PS/2 mouse plugged in.
> */
> static void __init reserve_ebda_region(void)
> {
> - unsigned int addr;
> - addr = get_bios_ebda();
> - if (addr)
> - reserve_bootmem(addr, PAGE_SIZE, BOOTMEM_DEFAULT);
> + unsigned int lowmem, ebda_addr;
> +
> + /* end of low (conventional) memory */
> + lowmem = *(unsigned short *)__va(BIOS_LOWMEM_KILOBYTES);
> + lowmem <<= 10;
> +
> + /* start of EBDA area */
> + ebda_addr = *(unsigned short *)__va(BIOS_EBDA_SEGMENT);
> + ebda_addr <<= 4;
> +
> + /* Fixup: bios puts an EBDA in the top 64K segment */
> + /* of conventional memory, but does not adjust lowmem. */
> + if ((lowmem - ebda_addr) <= 0x10000)
> + lowmem = ebda_addr;
> +
> + /* Fixup: bios does not report an EBDA at all. */
> + /* Some old Dells seem to need 4k anyhow (bugzilla 2990) */
> + if ((ebda_addr == 0) && (lowmem >= 0x9f000))
> + lowmem = 0x9f000;
> +
> + /* Paranoia: should never happen, but... */
> + if (lowmem >= 0x100000)
> + lowmem = 0xa0000;
> +
> + /* reserve all memory between lowmem and the 1MB mark */
> + reserve_bootmem(lowmem, 0x100000 - lowmem, BOOTMEM_DEFAULT);
> }
>
> #ifndef CONFIG_NEED_MULTIPLE_NODES
> @@ -619,16 +651,9 @@ void __init setup_bootmem_allocator(void)
> */
> reserve_bootmem(0, PAGE_SIZE, BOOTMEM_DEFAULT);
>
> - /* reserve EBDA region, it's a 4K region */
> + /* reserve EBDA region */
> reserve_ebda_region();
>
> - /* could be an AMD 768MPX chipset. Reserve a page before VGA to prevent
> - PCI prefetch into it (errata #56). Usually the page is reserved anyways,
> - unless you have no PS/2 mouse plugged in. */
> - if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD &&
> - boot_cpu_data.x86 == 6)
> - reserve_bootmem(0xa0000 - 4096, 4096, BOOTMEM_DEFAULT);
> -
> #ifdef CONFIG_SMP
> /*
> * But first pinch a few for the stack/trampoline stuff
>
--
Ian Campbell

In spite of everything, I still believe that people are good at heart.
-- Anne Frank