[vbox-dev] Question regarding channel callbacks between guest and host

Michael Thayer michael.thayer at oracle.com
Thu Apr 24 14:00:00 GMT 2014

Hello Jaeyong,

On 24/04/14 15:18, Jaeyong Yoo wrote:
> While I'm reading the source, I found the following sequence of
> function call:
> [In xpdm directory] VBoxMPHGSMIDpc () -->
>                               VBoxHGSMIProcessHostQueue ()  -->
>                               hgsmiHostCommandQueryProcess() -->
>                               hgsmiHostCmdProcess () -->
>                               HGSMIBufferProcess () -->
>                               hgsmiBufferProcess () -->
>                               HGSMIChannelHandlerCall ()  -->
>                               pfnHandler()   ==
> [Host side]             vbvaChannelHandler
> It looks like a guest-side function (in VGA driver) VBoxMPHGSMIDpc
> calls the host-side function (in VGA emulator) vbvaChannelHandler.
> How is this possible since host and guest are in different address
> space (if we are using nested paging table)? Or, I'm feeling like I
> miss something.

I'm not too familiar with that code, but what makes you think that 
pfnHandler points to vbvaChannelHandler() inside the guest driver?  See 
line 626 of 

   rc = HGSMIChannelRegister(&pCommon->hostCtx.channels, u8Channel,
                             "VGA Miniport HGSMI channel",
                             pContexts, &s_OldHandler);

So I would guess that VBoxVbvaChannelGenericHandlerCB() is what is being 
called.  In any case there should be no direct calls from the guest to 
the host address space.


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