Re: [Xen-devel] Requesting a freeze exception for vm_event memory introspection helpers

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 07/13/2015 05:56 PM, Wei Liu wrote:
> On Mon, Jul 13, 2015 at 05:42:48PM +0300, Razvan Cojocaru wrote:
>> On 07/13/2015 05:25 PM, Wei Liu wrote:
>>> On Mon, Jul 13, 2015 at 05:10:21PM +0300, Razvan Cojocaru wrote:
>>>> On 07/13/2015 04:51 PM, Wei Liu wrote:
>>>>> On Mon, Jul 13, 2015 at 10:35:44AM +0300, Razvan Cojocaru wrote:
>>>>>> Hello,
>>>>>>
>>>>>> I'd like to ask for a freeze exception for the "vm_event memory
>>>>>> introspection helpers" series.
>>>>>>
>>>>>> [PATCH 1/3] xen/mem_access: Support for memory-content hiding
>>>>>> [PATCH 2/3] xen/vm_event: Support for guest-requested events
>>>>>> [PATCH 3/3] xen/vm_event: Deny register writes if refused by
>>>>>> vm_event reply
>>>>>>
>>>>>> All patches have been acked by at least one person (though patch 1 is
>>>>>> still under some discussion).
>>>>>>
>>>>>> 1. Benefits of the series making it in this release:
>>>>>>
>>>>>> * Probably the most important benefit is that 4.6 development has been
>>>>>> very open to refactoring vm_events, and patch 3/3 makes vm_events behave
>>>>>> in a consistent manner (all register-write vm_events are pre-write 
>>>>>> events).
>>>>>>
>>>>>> * There are 3rd parties interested in these features (Tamas, for
>>>>>> example, has already expressed interest in uses of patch 1/1).
>>>>>>
>>>>>
>>>>> These aren't really good arguments for getting this in for 4.6. It would
>>>>> be easy for you and Tamas to carry three patches for a while.
>>>>>
>>>>> What are the impact to end users (end users -- meaning either system
>>>>> administrator that sets up Xen and other developers who want to use the
>>>>> new interface you introduce)?  Does this series consist of the last
>>>>> missing pieces of a feature?
>>>>
>>>> Not just missing pieces of a feature, the guest-requested (formerly
>>>> VMCALL) vm_event patch is crucial for memory introspection, and without
>>>> the first patch it's very difficult to resume monitoring Windows guests
>>>> that have been put in hybernate-mode (instead of powered off).
>>>>
>>>
>>> Right, so with these three patches, memory introspection is a complete
>>> feature (as in, to cover most use cases)? Or is it still a work in
>>> progress even after these patches are merged?
>>
>> Indeed, with these three patches x86 memory introspection is a complete
>> feature. There are still tweaks, of course, such as the ones mentioned
>> before (ARM support would be nice, some optimizations for REP emulation,
>> extending the emulator to be able to handle everything, etc.), but the
>> important part would be complete with these.
>>
>>>> I don't follow the impact question, sorry. If an administrator doesn't
>>>> use these features, the impact (in terms of speed and resources) should
>>>> be next to nothing.
>>>>
>>>
>>> OK, let me be specific. If I'm to build a product by either using Xen as
>>> out-of-box virtualisation platform or building my own software on top of
>>> Xen, would it make any *big* difference if I have your three patches?
>>
>> I would say yes, and this from experience. I've been maintaining a
>> series of patches internally for a while now (I think since Xen 4.0),
>> and I've come across two main categories of non-trivial issues:
>>
>> 1. Things tend to change a lot with Xen, especially lately, so at least
>> for individual patches it has come more often than I would have thought
>> to almost complete rewrites when going up to the next Xen version.
>>
>> 2. Even with a lot of work and testing, I still think a patch that has
>> been reviewed for a few months on xen-devel is better that an in-house
>> one. Xen is one of the best written and most elegant C projects I've
>> seen, but it's still evolving quickly and largely undocumented, so
>> there's always the chance a lone developer will miss something somewhere.
>>
>> So yes, again, I'd say that being able to use Xen out-of-the box for
>> introspection is the (much) better choice, at least in my experience.
>>
>>>> If a freeze exception for the whole series is not an option at this
>>>> point, would it be appropriate to just drop patches requiring more
>>>> review and get the more acked ones in?
>>>>
>>>
>>> I'm not saying it's not possible. I'm trying to make sense of what is
>>> going on at the moment.
>>>
>>> Maybe other maintainers can help enlighten me and express their
>>> preference? And I guess with your maintainer's hat on it would be a
>>> "yes, let's merge them" from you. :-)
>>
>> I understand. Thanks for having this conversation!
>>
> 
> Thanks for your clarification. If you can get your next version all
> acked /reviewed I would be fine with it going in.  Do note that the cut
> off day for applying patches with freeze exception is next Friday.

Thank you, I appreciate it! Duly noted.


Thanks,
Razvan

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