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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan



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