Re: [Xen-devel] What function pointer does %%gs contain?

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

The function address that is pushed by the individual fault handlers
before they call the common error_code path. It's a gross abuse of the
GS symbolic constant. :-)

Try running through one of the fault paths by hand, writing down the
stack state at each step...

 HTH,
 Keir

> I'm searching Xen's source code to understand exception handling. I've
> pasted a piece of code below.
> 
> In arch/x86/x86_32/entry.S:
> error_code:
>       pushl %fs
>       pushl %es
>       pushl %ds
>       pushl %eax
>       xorl  %eax,%eax
>       pushl %ebp
>       pushl %edi
>       pushl %esi
>       pushl %edx
>       decl  %eax                      # eax = -1
>       pushl %ecx
>       pushl %ebx
>       cld
>       movl  %gs,%ecx
>       movl  ORIG_EAX(%esp), %esi      # get the error code
>       movl  GS(%esp), %edi            # get the function address
>       movl  %eax, ORIG_EAX(%esp)
>       movl  %ecx, GS(%esp)
>       movl  $(__HYPERVISOR_DS),%edx
>       movl  %edx,%ds
>       movl  %edx,%es
>       movl  %edx,%fs
>       movl  %edx,%gs
>       movl  %esp,%edx
>       pushl %esi                      # push the error code
>       pushl %edx                      # push the pt_regs pointer
>       GET_CURRENT(%ebx)
>       call  *%edi
>         addl  $8,%esp
>         movb  CS(%esp),%al
>       testb $3,%al
>       je    restore_all_xen
>         jmp   process_guest_exception_and_events
> 
> But I can not understand what function does following sentence refer to
>       movl  GS(%esp), %edi            # get the function address
> Where does it come from?
> 
> Thanks,
> Benjamin
> 
> 
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 -=- MIME -=- 
I'm searching Xen's source code to understand exception handling. I've
pasted a piece of code below.

In arch/x86/x86_32/entry.S:
error_code:
        pushl %fs
        pushl %es
        pushl %ds
        pushl %eax
        xorl  %eax,%eax
        pushl %ebp
        pushl %edi
        pushl %esi
        pushl %edx
        decl  %eax                      # eax =3D -1
        pushl %ecx
        pushl %ebx
        cld
        movl  %gs,%ecx
        movl  ORIG_EAX(%esp), %esi      # get the error code
        movl  GS(%esp), %edi            # get the function address
        movl  %eax, ORIG_EAX(%esp)
        movl  %ecx, GS(%esp)
        movl  $(__HYPERVISOR_DS),%edx
        movl  %edx,%ds
        movl  %edx,%es
        movl  %edx,%fs
        movl  %edx,%gs
        movl  %esp,%edx
        pushl %esi                      # push the error code
        pushl %edx                      # push the pt_regs pointer
        GET_CURRENT(%ebx)
        call  *%edi
        addl  $8,%esp
        movb  CS(%esp),%al
        testb $3,%al
        je    restore_all_xen
        jmp   process_guest_exception_and_events

But I can not understand what function does following sentence refer to
        movl  GS(%esp), %edi            # get the function address
Where does it come from?

Thanks,
Benjamin


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