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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan



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