Re: [Xen-devel] [PATCH 2/2] altp2m: Implement p2m_get_mem_access for altp2m views

["Lengyel, Tamas" <tlengyel@xxxxxxxxxxx>]

On Jan 28, 2016 8:02 AM, "Razvan Cojocaru" <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>
> On 01/28/2016 04:42 PM, Lengyel, Tamas wrote:
> >
> > On Jan 28, 2016 6:38 AM, "Jan Beulich" <JBeulich@xxxxxxxx
> > <mailto:JBeulich@xxxxxxxx>> wrote:
> >>
> >> >>> On 27.01.16 at 21:06, <tlengyel@xxxxxxxxxxx
> > <mailto:tlengyel@xxxxxxxxxxx>> wrote:
> >> > --- a/xen/arch/x86/mm/p2m.c
> >> > +++ b/xen/arch/x86/mm/p2m.c
> >> > @@ -1572,7 +1572,9 @@ void p2m_mem_access_emulate_check(struct vcpu *v,
> >> >Â Â Â Â Â bool_t violation = 1;
> >> >Â Â Â Â Â const struct vm_event_mem_access *data = ""> > >> >
> >> > -Â Â Â Â if ( p2m_get_mem_access(v->domain, _gfn(data->gfn),
> > &access) == 0 )
> >> > +Â Â Â Â if ( p2m_get_mem_access(v->domain, _gfn(data->gfn),
> >> > +Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â altp2m_active(v->domain) ?
> > vcpu_altp2m(v).p2midx : 0,
> >> > +Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â &access) == 0 )
> >>
> >> This looks to be a behavioral change beyond what title and
> >> description say, and it's not clear whether that's actually the
> >> behavior everyone wants.
> >
> > I'm fairly comfident its exactly the expected behavior when one uses
> > mem_access in altp2m tables and emulation. Right now because the lack of
> > this AFAIK emulation would not work correctly with altp2m. But Razvan
> > probably can chime in as he uses this path actively.
>
> I've done an experiment to see how much slower using altp2m would be as
> compared to emulation - so I'm not a big user of the feature, but I did
> find it cumbersome to have to work with two sets of APIs (one for what
> could arguably be called the default altp2m view, i.e. the regular
> xc_set_mem_access(), and one for altp2m, i.e.
> xc_altp2m_set_mem_access()). Furthermore, the APIs do not currently
> offer the same features (most notably, xc_altp2m_get_mem_access() is
> completely missing). I've mentioned this to Tamas while initially trying
> to get it to work.
>
> Now, whether the behaviour I expect is what everyone expects is, of
> course, wide open to debate. But I think we can all agree that the
> altp2m interface can, and probably should, be improved.
>

There is that, but also, what is the exact logic behind doing this check before emulation? AFAIU emulation happens in response to a vm_event so we should be fairly certain that this check succeeds as it just verifies that indeed the permissions are restricted by mem_access in the p2m (and with altp2m this should be the active one). But when is this check normally expected to fail?

Tamas

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