Xen devel - May 2013 - What's a good way of skipping an instruction for a HVM guest?

Hello,

how do I skip the current instruction from the HV? That is, how do I
know what instruction length to add to guest_cpu_user_regs()->eip to
achieve this?

Could this be done by hvm_emulate_one() with a copy of
guest_cpu_user_regs() and then setting guest_cpu_user_regs()->eip to
the eip in the hvmemul_ctxt? Would this have side-effects?


Thanks,
Razvan Cojocaru

On 16/05/2013 16:04, "Razvan Cojocaru" <rzvncj@gmail.com> wrote:
> Hello,
> 
> how do I skip the current instruction from the HV? That is, how do I
> know what instruction length to add to guest_cpu_user_regs()->eip to
> achieve this?
> 
> Could this be done by hvm_emulate_one() with a copy of
> guest_cpu_user_regs() and then setting guest_cpu_user_regs()->eip to
> the eip in the hvmemul_ctxt? Would this have side-effects?
You would also need to provide a set of x86_emulate_ops that are side-effect
free (e.g., to avoid modifying CRn, DRn, memory, MSRs, ...).

There is no other fully generic way to do it though. VMX provides the
trapping instruction length on vmexit, but not in all cases. Our SVM code
also has a limited instruction-length helper function, but only for a few
instructions for which it is required.

 -- Keir
> 
> Thanks,
> Razvan Cojocaru
> 
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