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Posted: **Fri Jun 26, 2020 4:29 pm**

nullplan wrote:I wrote the syscall handler first, and then the multitasking code afterwards. The syscall handler was basically an outgrowth of the interrupt handler code I needed first.

That's interesting... I came from a different direction where I implemented interrupt handlers first, multitasking second and system calls last.

The end result is that I use a fake interrupt frame to spawn kernel thread and to jump to user space.

For example:

Code: Select all

.global JumpToUserMode
JumpToUserMode:

    # rdi = entry point in user space
    # rsi = user args
    # rdx = user space stack

    movq $0x1b, %rax    # Selector 0x18 (User Data) + RPL 3
    movw %ax, %ds
    movw %ax, %es

    # Build a fake iret frame
    pushq %rax          # Selector 0x18 (User Data) + RPL 3
    pushq %rdx          # User space stack
    pushq $0x202        # rflags = interrupt enable + reserved bit
    pushq $0x23         # Selector 0x20 (User Code) + RPL 3
    pushq %rdi          # Entry point in user space

    # Setup "user args" as parameter to user function
    movq %rsi, %rdi     # First parameter = user args

    # Interrupts need to be disabled to ensure we don't
    # get interrupted between swapgs and iret.
    cli

    swapgs
    iretq

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TSS and software switching

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