Re: [Xen-devel] Implementing mem-access for PV guests

["Aravindh Puthiyaparambil (aravindp)" <aravindp@xxxxxxxxx>]

> -----Original Message-----
> From: Tim Deegan [mailto:tim@xxxxxxx]
> Sent: Thursday, April 25, 2013 2:55 AM
> To: Aravindh Puthiyaparambil (aravindp)
> Cc: Cutter 409; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: Implementing mem-access for PV guests
> 
> At 01:02 +0000 on 25 Apr (1366851740), Aravindh Puthiyaparambil (aravindp)
> wrote:
> > > > I'm finally to a point where I can start looking at this more closely.
> > > > I'm trying to wrap my head around the shadow code to figure out
> > > > the right course of action.
> > > >
> > > > I'd want HVMOP_set_mem_access to work with both shadow and EPT,
> so
> >
> > Getting this to work with shadow would allow non-NPT HVM guests to
> > have mem_access support or will this also extend to PV guests?
> 
> It could extend to PV guests as long as you're willing to run them on shadow
> pagetables (i.e. with a significant performance hit).

Given that  64-bit PV already takes a performance hit from syscalls, I would 
rather not taken the added hit with shadow PTs.
 
> > I am interested in getting this to work for PV guests so I was
> > wondering how much extra work that would be. I can definitely help out
> > with this effort.
> 
> I think you'd have to take a good look at the hypercall interface -- PV guests
> have more ways of causing the hypervisor to read and write memory for
> them (e.g. the MMU ops) which wouldn't be intercepted by shadow PTs.
> It certainly ought to be possible, though.

OK, I will take a look at the hypercall interface. Does any shadowing come in 
to the picture with a PV guest when using a modern pv_ops? 

Thanks,
Aravindh

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