Virtual vs physical address in kmalloc

[quadrant]

I am following James Molloy's Kernel tutorial. In it, he has this bit of code for kmalloc:

Code: Select all

u32int kmalloc(u32int sz, int align, u32int *phys)
{
  if (align == 1 && (placement_address & 0xFFFFF000)) // If the address is not already page-aligned
  {
    // Align it.
    placement_address &= 0xFFFFF000;
    placement_address += 0x1000;
  }
  if (phys)
  {
    *phys = placement_address;
  }
  u32int tmp = placement_address;
  placement_address += sz;
  return tmp;
}

Accompanied with the description:

kmalloc will return a virtual address. But, we also (bear with me, you'll be glad we did later) need to get the physical address of the memory allocated.

I have rewritten the code snippet as follows:

Code: Select all

u32int kmalloc_ ( u32int size, int align, u32int *physical_address )
{
	// Page align the address
	if ( align == 1 && ( placement_address & 0xFFFFF000 ) )
	{
		placement_address &= 0xFFFFF000;
		placement_address += 0x1000;
	}

	u32int base = placement_address;

	// Share physical address
	if ( physical_address )
	{
		*physical_address = base;
	}

	placement_address += size;

	return base;  // how is this different from physical_address?
}

It seems to me that the virtual address returned is the same as the physical address shared. So why make the distinction?

[Brendan]

Hi,

I am following James Molloy's Kernel tutorial.

It seems to me that the virtual address returned is the same as the physical address shared. So why make the distinction?

To be "perfect", a tutorial needs to be simple enough for beginners to learn from but also needs to be complex enough to cover all useful information. These requirements are mutually exclusive (it's impossible for a tutorial to be simple enough and complex enough at the same time) and therefore tutorials must be "bad" in some way; so almost all tutorials aim to be simple enough (and therefore deliberately fail to cover anything complex enough to be useful in practice).

The distinction is made because in a (more complex) real operating system the virtual address would be different to the physical address.


Cheers,

Brendan

[quadrant]

Ah ok, thanks!