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

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> 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.

Jan



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