'repz' instruction not supported.

Ashutosh Kumar mrashutosh at hotmail.com
Fri Nov 21 10:08:15 UTC 2014

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


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


> Date: Fri, 21 Nov 2014 00:08:15 -0800
> Subject: Re: 'repz' instruction not supported.
> From: neelnatu at gmail.com
> To: mrashutosh at hotmail.com
> CC: freebsd-virtualization at freebsd.org
> Hi Ashutosh,
> On Wed, Nov 19, 2014 at 10:30 PM, Ashutosh Kumar <mrashutosh at 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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