What's mapped in the lower half? #470

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1. Freax13 commented on Nov 1, 2024

   

   Freax13

   on Nov 1, 2024

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   That's probably the context switch function of the boot loader:
   

   bootloader/common/src/lib.rs

   Lines 235 to 260 in ea3f61a

   	// identity-map context switch function, so that we don't get an immediate pagefault
   	// after switching the active page table
   	let context_switch_function = PhysAddr::new(context_switch as *const () as u64);
   	let context_switch_function_start_frame: PhysFrame =
   	PhysFrame::containing_address(context_switch_function);
   	for frame in PhysFrame::range_inclusive(
   	context_switch_function_start_frame,
   	context_switch_function_start_frame + 1,
   	) {
   	let page = Page::containing_address(VirtAddr::new(frame.start_address().as_u64()));
   	match unsafe {
   	// The parent table flags need to be both readable and writable to
   	// support recursive page tables.
   	// See https://github.com/rust-osdev/bootloader/issues/443#issuecomment-2130010621
   	kernel_page_table.map_to_with_table_flags(
   	page,
   	frame,
   	PageTableFlags::PRESENT,
   	PageTableFlags::PRESENT | PageTableFlags::WRITABLE,
   	frame_allocator,
   	)
   	} {
   	Ok(tlb) => tlb.flush(),
   	Err(err) => panic!("failed to identity map frame {:?}: {:?}", frame, err),
   	}
   	}

   
   These pages contain a function that switches to the kernel page tables and jumps to its entry point. You can safely unmap it.

   See also #240
2. mrjbom commented on Nov 1, 2024

   

   mrjbom

   on Nov 1, 2024

   Last edited by mrjbom

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   If it turns out not to be context switch function, then I suppose I can safely unmap these addresses anyway? Why does the bootloader leave these things mapped?
   For example, GDT could be safely unmapped by bootloader.
3. Freax13 commented on Nov 1, 2024

   

   Freax13

   on Nov 1, 2024

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   If it turns out not to be context switch function, then I suppose I can safely unmap these addresses anyway?

   I'm pretty sure it's the context switch function. If it's not the context switch function, then it depends on the memory for whether it's safe to unmap.

   Why does the bootloader leave these things mapped?

   The context switch function needs to do two things:

   1. Switch to the kernel's page tables.
   2. Jump to the kernel's entry point.

   It needs to do these steps in that order. The problem is that if we don't map the context switch function into the kernel's page tables, we can't do the second step after switching the page tables.

   For example, GDT could be safely unmapped by bootloader.

   The GDT can't be safely unmapped before switching to a new one. Some instructions like iret read from the GDT and if the GDT is no longer mapped, this causes a page fault.

   👍React with 👍1Reacted by mrjbom
4. mrjbom

   closed this as completedon Nov 1, 2024