Re: [Xen-devel] [PATCH 0/5] xen: better grant v2 support

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.08.17 at 09:49, <jgross@xxxxxxxx> wrote:
> On 22/08/17 14:48, Jan Beulich wrote:
>>>>> On 21.08.17 at 20:05, <jgross@xxxxxxxx> wrote:
>>> Currently Linux has no support for grant v2 as this would reduce the
>>> maximum number of active grants by a factor of 2 compared to v1,
>>> because the number of possible grants are limited by the allowed number
>>> of grant frames and grant entries of v2 need twice as much bytes as
>>> those of v1.
>>>
>>> Unfortunately grant v2 is the only way to support either guests with
>>> more than 16TB memory size or PV guests with memory above the 16TB
>>> border, as grant v1 limits the frame number to be 32 bits wide.
>>>
>>> In order to remove the disadvantage of grant v2 this patch series
>>> enables configuring different maximum grant frame numbers for v1 and
>>> v2.
>> 
>> But that does imply higher memory footprint of such a guest in Xen,
>> doesn't it?
> 
> With current defaults this would need up to 128kB more for a guest using
> v2 grants.

At least in an auto-ballooned setup this may make the difference
between a guest being able or failing to start.

>> The limit, after all, is there to bound resource use of
>> DomU-s.  I wonder whether we shouldn't make any such increase
>> dependent on first putting in place proper accounting of the memory
>> used for individual domains.
> 
> So you would want to have a way to count pages (or bytes?) allocated for
> hypervisor internal needs on a per-domain basis, right?
> 
> Would that be additional to struct domain -> xenheap_pages or would you
> want to merge the new counter into it? I guess a new field would be
> required in order to avoid counting some data twice.
> 
> Do you have an idea what to do with that value? Do you want to expose it
> to the user (dom0 admin), or should it be used just inside the
> hypervisor and e.g. printed by a debug key handler?
> 
> Do you want an additional set of allocating functions doing the
> accounting, or should the existing functions be used with an additional
> domain pointer, or should the caller be responsible doing the additional
> accounting?
> 
> Do you want an all-or-nothing approach or a gradual move to add the new
> accounting step by step?

We've been vaguely discussing this in the past on a few occasions.
My personal thinking is that the "memory=" setting in a guest config
really ought to express all the memory associated with a guest. But
of course there'll be problems with us starting to do so, and that's
beyond people observing less memory in their guests. Switching to
such a full accounting model will require some careful thought (and
discussion up front). Hence I've only said "I wonder whether", i.e.
I don't mean to make this a strict prerequisite to the proposed
changes here. I'd be in particular interested to hear opinions of a
few other people.

Jan


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