Re: [ros-dev] [ros-diffs] [sir_richard] 45052: Patch that fixes VMWare boot (and should fix QEMu/KVM boot on the testbot): [NTOS]: A trap can get us into a state where DS/ES are invalid, making any pointer dereference (on DS/ES segmented memory, not SS, the stack) crash (and probably double-fault). Therefore, we have to be careful to switch to a good DS/ES before touching the TrapFrame pointer, which we don't have in ESP like the ASM code, but in a DS/ES-segmented register. For V8086 traps we can switch to

[Timo Kreuzer]

Hi,

I suggest the following changes to the current implementation:

1.) Replace the PUSHA in the trap entry code, by a series of MOVs to the
correct stack positions. The same for the trap exit code.
KiTrapFrameFromPushaStack can be removed then. This would result in more
clear, less complex and faster code, with only  a few additional
assembly instructions in  the trap entry macro. The exit could be done
either by normal call/return or by a jmp to an exit handler.

2.) Segements should be fixed up before entering C code, as not doing so
may introduce possible compiler dependend breakages. It's also much
cleaner and there's no reason to do the same stuff later in inline
assembly instead of direcly in the asm entry point.

The resulting code might looks something like this:

    /* Allocate KTRAP_FRAME */
    sub esp, KTRAP_FRAME_LENGTH - 10 * 4

    /* Save integer registers */
    mov [esp + KTRAP_FRAME_EBP], ebp
    mov [esp + KTRAP_FRAME_EBX], ebx
    mov [esp + KTRAP_FRAME_ESI], esi
    mov [esp + KTRAP_FRAME_EDI], edi
    mov [esp + KTRAP_FRAME_EAX], eax
    mov [esp + KTRAP_FRAME_ECX], ecx
    mov [esp + KTRAP_FRAME_EDX], edx
    mov [esp + KTRAP_FRAME_EBX], ebx

    /* Save segment regs */
    mov [esp + KTRAP_FRAME_SEGDS], ds
    mov [esp + KTRAP_FRAME_SEGES], es

    /* Fixup segment regs */
    mov ax, KGDT_R3_DATA | RPL_MASK
    mov ds, ax
    mov es, ax


Timo