RE: temp. mem mappings

cohutta
2001-06-06T21:14:26Z

>===== Original Message From "Stephen C. Tweedie" <sct@redhat.com> =====
>Hi,
>
>On Tue, Jun 05, 2001 at 04:42:52PM -0400, cohutta wrote:
>
>> I don't really want to play with the page tables if i can help it.
>> I didn't use ioremap() because it's real system memory, not IO bus
>> memory.
>>
>> How much normal memory is identity-mapped at boot on x86?
>> Is it more than 8 MB?
>
>> I'm trying to read some ACPI tables, like the FACP.
>> On my system, this is at physical address 0x3fffd7d7 (e.g.).
>
>It depends at what time during boot. Some ACPI memory is reusable
>once the system boots: the kernel parses the table then frees up the
>memory which the BIOS initialised.
>
>VERY early in boot, while the VM is still getting itself set up, there
>is only a minimal mapping set up by the boot loader code. However,
>once the VM is initialised far enough to let you play with page
>tables, all memory will be identity-mapped up to just below the 1GB
>watermark.

I think this is part of the problem: on my 1 GB system, the
ACPI tables are at physical 0x3fffxxxx == virtual 0xffffxxxx,
which could conflict with the APIC and IOAPIC mappings
(from fixmap.h).
I removed 256 MB, but i still have a few problems.

>> kmap() ends up calling set_pte(), which is close to what i am
>> already doing. i'm having a problem on the unmap side when i
>> am done with the temporary mapping.
>
>kunmap(). :-) But kmap only works on CONFIG_HIGHMEM kernel builds.
>On kernels built without high memory support, kmap will not allow you
>to access memory beyond the normal physical memory boundary.

Well, i'm talking about physical memory, but it's marked as ACPI
data.

Another part of the problem is that I need to do this early in
arch/i386/kernel/setup.c::setup_arch(), like between calls to
paging_init() and init_apic_mappings(). I can't use ioremap()
here can i? ioremap() calls get_vm_area() which calls
kmalloc(), and i don't think i can use kmalloc() just yet.

methinks that i'm back to a modification of Timur's suggestion--
a bunch of manual page dir/table changes.

Any other suggestions or corrections to my comments?

thanks.