Re: [Xen-devel] [PATCH v2] x86/mm: Add mem access rights to NPT

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

On Vi, 2018-06-29 at 00:13 -0600, Jan Beulich wrote:
> >
> > >
> > > >
> > > > Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx> 06/28/18 4:53
> > > > PM >>>
> > On Jo, 2018-06-28 at 08:40 -0600, Jan Beulich wrote:
> > >
> > > >
> > > > >
> > > > > >
> > > > > > On 28.06.18 at 16:10, <aisaila@xxxxxxxxxxxxxxx> wrote:
> > > > On Vi, 2018-06-22 at 09:51 -0600, Jan Beulich wrote:
> > > > >
> > > > > >
> > > > > > >
> > > > > > > >
> > > > > > > > On 18.06.18 at 17:17, <aisaila@xxxxxxxxxxxxxxx> wrote:
> > > > > > From: Isaila Alexandru <aisaila@xxxxxxxxxxxxxxx>
> > > > > >
> > > > > > This patch adds access rights for the NPT pages. The access
> > > > > > rights
> > > > > > are
> > > > > > saved in a radix tree with the root saved in p2m_domain.
> > > > > Sounds resource intensive. How many nodes would such a radix
> > > > > tree
> > > > > have
> > > > > on average?
> > > > The average is around 1478890 for a machine with 4GB of ram.
> > > Is this with ...
> > >
> > > >
> > > > >
> > > > > >
> > > > > > +static void p2m_set_access(struct p2m_domain *p2m,
> > > > > > unsigned
> > > > > > long
> > > > > > gfn,
> > > > > > +                                      p2m_access_t a)
> > > > > > +{
> > > > > > +    int rc;
> > > > > > +
> > > > > > +    if ( p2m_access_rwx == a )
> > > > > > +        radix_tree_delete(&p2m->mem_access_settings, gfn);
> > > > > > +
> > > > > > +    rc = radix_tree_insert(&p2m->mem_access_settings, gfn,
> > > > > > +                           radix_tree_int_to_ptr(a));
> > > > > Is there an "else" missing above here? Otherwise why would
> > > > > you
> > > > > delete the node first?
> > > > Yes it needs a else or a return. We plan not to have the rwx in
> > > > the
> > > > tree so we ca save up some space.
> > > ... this corrected? Otherwise I'm tempted to say that the
> > > creation of
> > > this radix tree needs to be avoided by all means, as long as it's
> > > not
> > > really needed.
> > The number was with this patch so no correction. This was done with
> > xen-access write and I don't think it will make a difference if you
> > change the access to all the mem pages.
> The question "on average" wasn't with xen-access in use. I'm worried
> of the
> overhead _without_ any introspection active.

I've started a win 7 machine with no introspection or xen-access. After
30 min of uptime there were 0 inserts into the tree. I guess the
overhead is down to a minimum with no user modified access rights.

Alex

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