Re: hvm_emulate_one() usage

[Razvan Cojocaru <rzvncj@gmail.com>]

> Hi,

Hello Tim, thank you for your answer!

> How about (from your user-space tool):
>   - use XEN_DOMCTL_DEBUG_OP_SINGLE_STEP_ON to enable single-stepping.
>   - respond to the mem_event that you're handling, causing the vcpu to be
>     unpaused.
> Then when the vcpu is scheduled, it will single-step in its own context,
> and you'll get another mem_event (assuming you've set
> HVM_PARAM_MEMORY_EVENT_SINGLE_STEP).  Once that happens:
>   - use XEN_DOMCTL_DEBUG_OP_SINGLE_STEP_OFF to disable single-stepping.
>   - respond to that event to unpause the vcpu.
>
> I guess if you're tyring to have some special case in the single-step
> handler that allows it to write to a page that it normally coudn't you
> might need to add an interface for controlling that.

Thanks, looks like something like that is the only way this would 
theoretically work. The problem is, for each allowed (emulated) write - 
which is the 'normal' case - there would be 3 dom0 <-> hypervisor 
roundtrips (2 fault mem_events and 1 single step mem_event). Since 
writes that need to be allowed do happen quite a lot, the domU would 
become very slow.

I was very much hoping to be able to do this with only one (page fault) 
mem_event per emulated write instruction.

Thanks,
Razvan Cojocaru