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x86_64 - disabling paging?
Posted: Sat Aug 27, 2011 6:42 am
by q3k
Hello,
I've been porting my microkernel to x86_64. For this, I need a function that will let me read raw memory data, by bypassing paging. Here's how I'm trying to implement it right now:
Code: Select all
asm_ReadU64Physical:
; disable paging
mov rcx, cr0;
btc rcx, 31;
mov cr0, rcx;
; do our stuff
mov rax, qword [rdi];
; enable paging
mov rdx, cr0
bts rdx, 31;
mov cr0, rdx
ret;
However, it seems to triple fault (I'm using this very early in my boot chain, almost right after switching to long mode, so I don't have any IDT or interrupt handlers set up yet) upon returning from the function (if I place a jmp $ right before ret, it just sits there). What am I doing wrong? The way I'm calling this code works, as dummy functions like these work and return correctly.
Code: Select all
asm_TestFunction:
mov rax, rdi;
add rax, 0x23;
ret
Any help would be very appreciated, as I am quite lost.
Re: x86_64 - disabling paging?
Posted: Sat Aug 27, 2011 6:56 am
by zity
You cannot disable paging once in long mode. If you disable paging in long mode, you will no longer be in long mode.
Re: x86_64 - disabling paging?
Posted: Sat Aug 27, 2011 7:07 am
by q3k
zity wrote:You cannot disable paging once in long mode. If you disable paging in long mode, you will no longer be in long mode.
*sigh*, that's what I feared, and kinda remembered from the Intel Manuals. Thanks for the clarification!
Re: x86_64 - disabling paging?
Posted: Sun Aug 28, 2011 1:49 am
by xyzzy
Since you have such a large virtual address space in long mode, you could just map all of physical memory into the kernel part of the address space somewhere at boot time and use that to access physical memory.
Re: x86_64 - disabling paging?
Posted: Sun Aug 28, 2011 3:56 pm
by Owen
xyzzy wrote:Since you have such a large virtual address space in long mode, you could just map all of physical memory into the kernel part of the address space somewhere at boot time and use that to access physical memory.
Virtual address space is 48-bits (with a big chasm down the middle). Physical address space is 56 bits (IIRC)
Re: x86_64 - disabling paging?
Posted: Sun Aug 28, 2011 4:27 pm
by stlw
Owen wrote:xyzzy wrote:Since you have such a large virtual address space in long mode, you could just map all of physical memory into the kernel part of the address space somewhere at boot time and use that to access physical memory.
Virtual address space is 48-bits (with a big chasm down the middle). Physical address space is 56 bits (IIRC)
There is no hardware supporting more than 46 physical bits in the market. Usually in it 40 bit only.
Re: x86_64 - disabling paging?
Posted: Sun Aug 28, 2011 5:07 pm
by Owen
stlw wrote:Owen wrote:xyzzy wrote:Since you have such a large virtual address space in long mode, you could just map all of physical memory into the kernel part of the address space somewhere at boot time and use that to access physical memory.
Virtual address space is 48-bits (with a big chasm down the middle). Physical address space is 56 bits (IIRC)
There is no hardware supporting more than 46 physical bits in the market. Usually in it 40 bit only.
AMD Family 10h (Phenom, Phenom II, 10h Opterons) all have a 48-bit physical address space. This will
just fit into your virtual address space, with a big divide down the middle...
Who knows what Bulldozer will bring.
Re: x86_64 - disabling paging?
Posted: Mon Aug 29, 2011 3:57 am
by xyzzy
That may be the maximum supported physical address space size, but it's unlikely that you'll encounter a system with that much RAM (for now, at least). What I do is set aside 256GB of the kernel address space to map physical memory to, and if something wants to access physical memory outside that a special mapping is created on the kernel heap for it.