Re: [PATCH v3 3/3] tools: introduce parameter max_wp_ram_ranges.

["Yu, Zhang" <yu.c.zhang@linux.intel.com>]

On 2/5/2016 1:12 AM, George Dunlap wrote:
> On 04/02/16 14:08, Jan Beulich wrote:
>>>>> On 04.02.16 at 14:33, <Ian.Jackson@eu.citrix.com> wrote:
>>> Jan Beulich writes ("Re: [Xen-devel] [PATCH v3 3/3] tools: introduce parameter
>>> max_wp_ram_ranges."):
>>>> On 04.02.16 at 10:38, <yu.c.zhang@linux.intel.com> wrote:
>>>>> So another question is, if value of this limit really matters, will a
>>>>> lower one be more acceptable(the current 256 being not enough)?
>>>>
>>>> If you've carefully read George's replies, [...]
>>>
>>> Thanks to George for the very clear explanation, and also to him for
>>> an illuminating in-person discussion.
>>>
>>> It is disturbing that as a result of me as a tools maintainer asking
>>> questions about what seems to me to be a troublesome a user-visible
>>> control setting in libxl, we are now apparently revisiting lower
>>> layers of the hypervisor design, which have already been committed.
>>>
>>> While I find George's line of argument convincing, neither I nor
>>> George are maintainers of the relevant hypervisor code.  I am not
>>> going to insist that anything in the hypervisor is done different and
>>> am not trying to use my tools maintainer position to that end.
>>>
>>> Clearly there has been a failure of our workflow to consider and
>>> review everything properly together.  But given where we are now, I
>>> think that this discussion about hypervisor internals is probably a
>>> distraction.
>>
>> While I recall George having made that alternative suggestion,
>> both Yu and Paul having reservations against it made me not
>> insist on that alternative. Instead I've been trying to limit some
>> of the bad effects that the variant originally proposed brought
>> with it. Clearly, with the more detailed reply George has now
>> given (involving areas where he is the maintainer for), I should
>> have been more demanding towards the exploration of that
>> alternative. That's clearly unfortunate, and I apologize for that,
>> but such things happen.
>>
>> As to one of the patches already having for committed - I'm not
>> worried about that at all. We can always revert, that's why the
>> thing is called "unstable".
>
> It looks like I should have been more careful to catch up on the current
> state of things before I started arguing again -- please accept my
> apologies.
>

In fact, I need to say thank you all for your patience and suggestions.
I'm thrilled to see XenGT is receiving so much attention. :)

> I see that patch 2/3 addresses the gpfn/io question in the commit
> message by saying, "Previously, a new hypercall or subop was suggested
> to map write-protected pages into ioreq server. However, it turned out
> handler of this new hypercall would be almost the same with the existing
> pair - HVMOP_[un]map_io_range_to_ioreq_server, and there's already a
> type parameter in this hypercall. So no new hypercall defined, only a
> new type is introduced."
>
> And I see that 2/3 internally separates the WP_RAM type into a separate
> rangeset, whose size can be adjusted separately.
>
> This addresses my complaint about the interface using gpfns rather than
> MMIO ranges as an interface (somewhat anyway).  Sorry for not
> acknowledging this at first.
>
> The question of the internal implementation -- whether to use RB tree
> rangesets, or radix trees (as apparently ARM memaccess does) or p2m
> types -- is an internal implementation question.  I think p2m types is
> long-term the best way to go, but it won't hurt to have the current
> implementation checked in, as long as it doesn't have any impacts on the
> stable interface.
>
> At the moment, as far as I can tell, there's no way for libxl to even
> run a version of qemu with XenGT enabled, so there's no real need for
> libxl to be involved.
>

I agree.

> The purpose of having the limit would putatively be to prevent a guest
> being able to trigger an exhaustion of hypervisor memory by inducing the
> device model to mark an arbitrary number of ranges as mmio_dm.
>
> Two angles on this.
>
> First, assuming that limiting the number of ranges is what we want:  I'm
> not really a fan of using HVM_PARAMs for this, but as long as it's not
> considered a public interface (i.e., it could go away or disappear and
> everything would Just Work), then I wouldn't object.
>
> Although I would ask: would it instead be suitable for now to just set
> the default limit for WP_RAM to 8196 in the hypervisor, since we do
> expect it to be tracking gpfn ranges rather than IO regions?  And if we

That is what we have suggesting in v9. But Jan proposed we leave this
option to the admin. And to some extent, I can understand his concern.

> determine in the future that more ranges are necessary, to then do the
> work of moving it to using p2m types (or exposing a knob to adjust it)?
>
> But (and this the other angle): is simply marking a numerical limit
> sufficient to avoid memory exhaustion? Is there a danger that after
> creating several guests, such that Xen was now running very low on
> memory, that a guest would (purposely or not) cause memory to be
> exhausted sometime further after boot, causing a system-wide DoS (or
> just general lack of stability)?
>

This worry sounds reasonable. So from this point of view, I guess value
of this limit does matter. Setting it to 256 will not cause any
trouble, but setting it to a huge one would make this limitation
useless. Previously, I had thought 8K is an acceptable one for XenGT to
run smoothly, and for Xen heap to not over consume its memory, but
seems this value is not small enough to be acceptable. :)

Yu