Re: use of saved_eip

[Nitin A Kamble <nitin.a.kamble-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

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Hi Laurent,
  Sorry for calling by alst name. You 1st & last both names are totally
new to me. Are these french names?
  I understand your explanation. I was worried about code getting
misplaced due to automatic merges.
-- 
Thanks & Regards,
Nitin
Open Source Technology Center, Intel Corporation
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On Wed, 2007-09-26 at 14:51 -0700, Laurent Vivier wrote:
> Kamble, Nitin A wrote:
> > Hi Vivier, Avi,
> 
> Hi Nitin,
> (BTW, my first name is Laurent)
> 
> >   In order to debug faulures in my tree, I was looking at the
> saved_eip
> > changes coming from your commit. I did not understand the use of
> > saved_eip properly. like why is it used in the emulation of the pop
> > instruction. Can you please help me understand it's usage?
> 
> in emulate_instruction(), we decode instructions and copy vcpu
> registers
> to ctxt (in x86_decode_insn()), then we really emulate the instruction
> (in x86_emulate_insn()).
> 
> In x86_emulate_insn(), if we have a REP prefix, we decrement ECX and
> set
> EIP to next instruction, then we try to emulate the instruction.
> If the emulation fails (because this is a MMIO for instance) we have
> to
> restore the initial values of ECX and EIP because we will re-enter in
> x86_emulate_insn() once the IO has been managed by Qemu and thus ECX
> is
> decremented again and EIP set to next instruction again.
> 
> And you are right: _we_don't_have_to_do_that_for_the_pop_instruction_,
> it's a mistake because the REP prefix hasn't been processed at this
> level, it is managed (ECX and EIP are modified) later.
> 
> So, you can remove from pop_instruction:
> 
> 1383                         if (c->rep_prefix) {
> 1384                                 c->regs[VCPU_REGS_RCX] =
> saved_rcx;
> 1385                                 c->eip = saved_eip;
> 1386                         }
> 
> Sorry for the inconvenience,
> 
> Laurent
> 
> 


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