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How can you map more than 256 TiB in long mode.
https://forum.osdev.org/viewtopic.php?f=1&t=52085		 Page 1 of 1
Author:  antoni [ Sun Sep 05, 2021 1:31 pm ]
Post subject:  How can you map more than 256 TiB in long mode.
I'm switching to BOOTBOOT. For a long time I developed my bottom half kernel but now I want to try a higher half kernel.

The problem is that it's code segment for example is mapped to 0xFFFFFFFFFFE02000. However, using 4-level paging you can only map up to 256 TiB (max. range 0x1000000000000) of memory!

For now I use this function to get physical address of some page:

Code:
intptr_t vm_get_physaddr(struct vm_space *vm_space, void *virtualaddr)
{
   uint64_t pml4 = vm_space->pml4;
   uint64_t ptindex = (uint64_t) virtualaddr / 0x1000 % 512;
   uint64_t pdindex = ((uint64_t) virtualaddr / 0x200000) % 512;
   uint64_t pdpindex = ((uint64_t) virtualaddr / 0x40000000) % 512;
   uint64_t pml4index = ((uint64_t) virtualaddr / 0x8000000000);
   uint64_t *entry = (uint64_t*) pml4;

   if (!(entry[pml4index] & 1))
      return -1;
   entry = (uint64_t*) (entry[pml4index] & ~0xFFF);

   if (!(entry[pdpindex] & 1))
      return -1;
   entry = (uint64_t*) (entry[pdpindex] & ~0xFFF);

   if (!(entry[pdindex] & 1))
      return -1;
   entry = (uint64_t*) (entry[pdindex] & ~0xFFF);

   if (!(entry[ptindex] & 1))
      return -1;
   entry = (uint64_t*) (entry[ptindex] & ~0xFFF);

   return entry;
}


I was able to successfully get address of first 2GiB (or more if available) of memory which are identity mapped in BOOTBOOT protocol. However, obviously this method did not work with higher addresses. I wonder how they were even mapped.
Author:  qookie [ Sun Sep 05, 2021 2:01 pm ]
Post subject:  Re: How can you map more than 256 TiB in long mode.
The 48-bit address is sign extended into 64-bits, so bits 63-48 are supposed to have the same value as bit 47, which turns 0x800000000000 into 0xFFFF800000000000, and so on. If you try accessing an address where bits 63-48 don't have the same value as bit 47, you'll get a general protection fault with error code 0.
Author:  iansjack [ Sun Sep 05, 2021 2:50 pm ]
Post subject:  Re: How can you map more than 256 TiB in long mode.
Check out "canonical address" in the Programmer's Manual.
Author:  antoni [ Sun Sep 05, 2021 2:55 pm ]
Post subject:  Re: How can you map more than 256 TiB in long mode.
I'm not sure how exactly this works... I can't access this same address in non-sign-extended form.

Code:
(gdb) x/b 0xffffffffffe07d40
0xffffffffffe07d40 <buf>:   0x57
(gdb) x/b 0xffffffe07d40
0xffffffe07d40:   Cannot access memory at address 0xffffffe07d40
(gdb)
Author:  nullplan [ Sun Sep 05, 2021 9:29 pm ]
Post subject:  Re: How can you map more than 256 TiB in long mode.
antoni wrote:
I'm not sure how exactly this works... I can't access this same address in non-sign-extended form.
Yes, because those would be non-canonical. The CPU actually checks the sign extension and generates #GP(0) if it is wrong.
Author:  iansjack [ Mon Sep 06, 2021 3:05 am ]
Post subject:  Re: How can you map more than 256 TiB in long mode.
0xffffffe07d40 = 0000 0000 0000 0000 1111 1111 1111 1111 1111 1111 1110 0000 0111 1101 0010 0000 in binary.

Bit 47 is 1. This is not extended to bits 48 - 63, which are all 0, so the address is not canonical, and raises an error when you attempt to access it.

0xffffffffffe07d40 = 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1110 0000 0111 1101 0010 0000 in binary.

Bit 47 is 1. This is extended to bits 48 - 63, which are all 1, so the address is canonical and can be accessed without an error.
Author:  Ethin [ Mon Sep 06, 2021 2:58 pm ]
Post subject:  Re: How can you map more than 256 TiB in long mode.
If you want to map 256-Tbytes, check for five-level paging and then set CR4.LA57. Then set up your PML5 table and fill it with 512 PML5Es.Then canonicalize your addresses by sign-extending bit 56 to bits 57-64 (if memory serves).
Author:  iansjack [ Tue Sep 07, 2021 1:55 am ]
Post subject:  Re: How can you map more than 256 TiB in long mode.
Restricting your OS to the latest Xeon processors is probably not a good move for a hobby OS.
Author:  klange [ Tue Sep 07, 2021 3:21 am ]
Post subject:  Re: How can you map more than 256 TiB in long mode.
It's also not what the OP is asking for...

Quote:
I was able to successfully get address of first 2GiB (or more if available) of memory which are identity mapped in BOOTBOOT protocol. However, obviously this method did not work with higher addresses. I wonder how they were even mapped.


You need to walk the page tables, just like the MMU does, to translate a virtual address to a physical address.
Author:  Octocontrabass [ Tue Sep 07, 2021 2:28 pm ]
Post subject:  Re: How can you map more than 256 TiB in long mode.
antoni wrote:
The problem is that it's code segment for example is mapped to 0xFFFFFFFFFFE02000. However, using 4-level paging you can only map up to 256 TiB (max. range 0x1000000000000) of memory!

You can map 256 TiB total, but it's split into two 128 TiB pieces. The lower 128TiB is addresses from 0 to 0x00007FFFFFFFFFFF, and the upper 128 TiB is addresses from 0xFFFF800000000000 to 0xFFFFFFFFFFFFFFFF. Your example address 0xFFFFFFFFFFE02000 is in the upper 128 TiB.

With 4-level paging, you can't map anything from 0x0000800000000000 to 0xFFFF7FFFFFFFFFFF. Addresses in that range will always be invalid.
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