Re: use of saved_eip

[Laurent Vivier <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org>]

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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