Re: [RFC] use realmode code to reserve end-of-conventional-memory to 1MB

Eduardo Habkost
2008-03-05T16:02:53Z

On Thu, Feb 28, 2008 at 11:16:49PM +0000, Ian Campbell wrote:
>
> The patch below seems like the right thing to do. It certainly boots in
> a domU without the DMI problem (without any of the other related patches
> such as Alexander's).
>
> However ddcprobe hangs when run -- need to investigate some more, vm86
> in general works ok (i.e. my vm86 test code passes).
>
> BTW Jeremy, the kernel doesn't use XENMEM_memory_map -- any reason other
> than it not being useful at the time? These days the tools can push an
> arbitrary e820 down for a guest which might be useful to support,
> although nothing interesting is done with it today.
>
> Ian.
>
> x86/xen: Construct e820 map with a hole between 640K-1M.
>
> Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Jeremy Fitzhardinge <jeremy@goop.org>
> ---
> arch/x86/xen/setup.c | 3 ++-
> 1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> index 3bad477..2341492 100644
> --- a/arch/x86/xen/setup.c
> +++ b/arch/x86/xen/setup.c
> @@ -38,7 +38,8 @@ char * __init xen_memory_setup(void)
> unsigned long max_pfn = xen_start_info->nr_pages;
>
> e820.nr_map = 0;
> - add_memory_region(0, PFN_PHYS(max_pfn), E820_RAM);
> + add_memory_region(0, LOWMEMSIZE(), E820_RAM);
> + add_memory_region(HIGH_MEMORY, PFN_PHYS(max_pfn)-HIGH_MEMORY, E820_RAM);

Won't this waste 300+ KB of Perfectly Good RAM? Or I understood it
incorrectly?

I am aware that it would take more work to tell all kernel code that it
shouldn't look for BIOS data on this region when running as a domU guest,
but it seems that it would be a better solution.

--
Eduardo