Xen devel - Apr 2005 - domu_debug support removed on march 25th from traps.c in -unstable

Why?

      -Kip

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The comments are pretty self-explanatory - but it looks like the trace
flag is no longer getting cleared on trap. Continue after
single-stepping no longer works. Any thoughts on why?

         -Kip



On 4/12/05, Kip Macy <kip.macy@gmail.com> wrote:
> Why?
> 
>       -Kip
>
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The code belonged in debugger.h so I moved it there.

  -- Keir


On 13 Apr 2005, at 06:47, Kip Macy wrote:
> The comments are pretty self-explanatory - but it looks like the trace
> flag is no longer getting cleared on trap. Continue after
> single-stepping no longer works. Any thoughts on why?
>
>          -Kip
>
>
>
> On 4/12/05, Kip Macy <kip.macy@gmail.com> wrote:
>> Why?
>>
>>       -Kip
>>

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No idea why the trace trap flag isn''t getting cleared? I can work at
around it at the application level but that is aesthetically
displeasing.

       -Kip  

On 4/13/05, Keir Fraser <Keir.Fraser@cl.cam.ac.uk>
wrote:
> The code belonged in debugger.h so I moved it there.
> 
>   -- Keir
> 
> 
> On 13 Apr 2005, at 06:47, Kip Macy wrote:
> 
> > The comments are pretty self-explanatory - but it looks like the trace
> > flag is no longer getting cleared on trap. Continue after
> > single-stepping no longer works. Any thoughts on why?
> >
> >          -Kip
> >
> >
> >
> > On 4/12/05, Kip Macy <kip.macy@gmail.com> wrote:
> >> Why?
> >>
> >>       -Kip
> >>
> 
>
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On 13 Apr 2005, at 18:48, Kip Macy wrote:
> No idea why the trace trap flag isn''t getting cleared? I can work
at
> around it at the application level but that is aesthetically
> displeasing.
You mean that the TF bit is not cleared by the domu_debug code when it 
should be? The correct place to fix this is in 
include/asm-x86/debugger.h in debugger_trap_entry().

In general (not using domu_debug) it is not Xen''s job to clear TF. We 
clear it when we ''bounce'' the exception into the guest OS, but
the
saved application stack frame will include EFLAGS with the TF bit still 
set. This matches real hardware.

  -- Keir


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> You mean that the TF bit is not cleared by the domu_debug code when it
> should be? The correct place to fix this is in
> include/asm-x86/debugger.h in debugger_trap_entry().
Yes. Continuing after single-stepping no longer works.
> In general (not using domu_debug) it is not Xen''s job to clear TF.
We
> clear it when we ''bounce'' the exception into the guest
OS, but the
> saved application stack frame will include EFLAGS with the TF bit still
> set. This matches real hardware.
No it isn''t Xen''s job to clear it under normal circumstances
as that
would break PTRACE_SINGLESTEP in user-space. However, it was what the
PDB code was doing and it seemed easy enough to clear it there to
emulate PTRACE_SINGLESTEP for the kernel.

I honestly don''t care where it happens, it is easy enough to clear the
TF from xc_ptrace.c. I just happened to be *extremely* tired last
night and was dealing with other issues, so I was frustrated when that
didn''t work. I''ll just incorporate my fix to xc_ptrace.c in
with the
FreeBSD fixes for -unstable.


             -Kip

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