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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