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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.07.18 at 10:01, <aisaila@xxxxxxxxxxxxxxx> wrote:
> On Lu, 2018-07-02 at 00:37 -0600, Jan Beulich wrote:
>> >
>> > >
>> > > >
>> > > > On 29.06.18 at 18:42, <tamas@xxxxxxxxxxxxx> wrote:
>> > On Fri, Jun 29, 2018 at 3:38 AM Jan Beulich <JBeulich@xxxxxxxx>
>> > wrote:
>> > >
>> > >
>> > > >
>> > > > >
>> > > > > >
>> > > > > > On 29.06.18 at 10:42, <aisaila@xxxxxxxxxxxxxxx> wrote:
>> > > > 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.
>> > > Thanks, this is helpful to know. In that case though, as said
>> > > before, I'd
>> > > like to ask to defer setting up of the radix tree to the point
>> > > where it
>> > > actually is going to be needed.
>> > Are you worried about the radix tree being present (ie
>> > radix_tree_init
>> > have been called) before it is used for mem_access? As I said
>> > earlier,
>> > a lookup on an empty radix tree is equivalent of a NULL check. I
>> > don't
>> > get how not having the tree initialized will be any faster then
>> > doing
>> > a lookup on an empty one.
>> No, the question is not about performance. The point is about
>> reassurance that the tree isn't going to be used in normal (non-
>> introspection) operation. If it suddenly and unknowingly became
>> used down the road, the resource consumption pattern of
>> domains may change quite significantly. Not setting up the tree
>> unless needed likely also helps review of the changes, as it'll be
>> necessary to make sure in the patch that it won't get accessed
>> without having been set up.
>>
> I can move the radix tree init to p2m_pt_set_entry() so that it is
> called on the first use or I can move it to vm_event_enable() and then
> return in every case that the root is NULL.
> 
> What would be the best way to handle this?

I'd prefer the second of the named options.

Jan



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