Re: [Xen-devel] hvm_emulate_one() usage

[Razvan Cojocaru <rzvncj@xxxxxxxxx>]

> > I was very much hoping to be able to do this with only one (page fault)
> > mem_event per emulated write instruction.
> I'm sure that can be done.  The trick is to make sure the emulation
> happens in the guest context (i.e. when the guest is scheduled).  You
> could do that by (e.g.) defining a new mem_access type 'single-step
> writes' where a write fault triggers a single-step emulation in the
> fault handler as well as an asynchronous mem-event.
That's what I'm doing now (albeit with the plain 
MEM_EVENT_REASON_VIOLATION) - I'm emulating the write in 
p2m_mem_access_check(), where I'm in the guest context, just before 
putting the mem_event in the ring buffer.
The problem is, I don't want to do that. :) I want to stop certain 
writes _before_ they happen, and emulating the write instruction there 
first performs the write, and then notifies dom0 userspace about it.
The ideal sequence would be: 1. notify userspace about a would-be write, 
2. get the reply from userspace, 3. only write if userspace said OK. The 
point is that I don't know if the write should be allowed to happen or 
not until userspace replies.
Thanks,
Razvan Cojocaru


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