Yes you are right 'repz' prefixes is added to the ‘cmps’ and 'scas' 
instructions.

The actual opcode is “F3 A5” which is ‘rep movs’ (disassembly is showing
‘repz’).

 

Yes the guest is emitting these instructions to access MMIO
(local apic)

RegardsAshutosh

> Date: Fri, 21 Nov 2014 00:08:15 -0800
> Subject: Re: 'repz' instruction not supported.
> From: neeln...@gmail.com
> To: mrashut...@hotmail.com
> CC: freebsd-virtualization@freebsd.org
> 
> Hi Ashutosh,
> 
> On Wed, Nov 19, 2014 at 10:30 PM, Ashutosh Kumar <mrashut...@hotmail.com> 
> wrote:
> > Hi
> > We have found that emulation of 'repz' instruction is not supported in 
> > bhyve. This is causing VM_EXIT for Guest OS. Do we have plans to add 
> > support for instructions like 'repz'.
> >
> 
> 'repz' is actually an instruction prefix. The two instructions with
> which 'repz' is allowed are 'cmps' and 'scas'. Both these instructions
> typically don't need to be emulated since they operate on strings in
> "regular" guest memory.
> 
> Is your guest emitting these instructions to access MMIO? Can you
> share an objdump with the offending instructions?
> 
> best
> Neel
> 
> > RegardsAshutosh
> > _______________________________________________
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