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

Not disputing you need shadows.

My point is that, for both PV and HVM, get page from gfn would be the way to catch grant and foreign mappings. Which mem access currently can't. IIRC.


On Apr 25, 2013 11:40 AM, "Tim Deegan" <tim@xxxxxxx> wrote:
At 10:48 -0400 on 25 Apr (1366886887), Andres Lagar-Cavilla wrote:
> > 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).
> >
> >> 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.
> Tim,
> wouldn't intercepting all calls to get_page_from_gfn ought to cover
> it?

I doubt it -- in the PV code, an MFN is and MFN and needs no
translation; get_page_from_gfn() &c are only called from code that's
common to both PV and HVM.

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