Re: [PATCH v4 3/3] x86/ioreq server: Add HVMOP to map guest ram with p2m_ioreq_server to an ioreq server.

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

On 6/22/2016 5:47 PM, George Dunlap wrote:
> On 22/06/16 10:29, Jan Beulich wrote:
>>>>> On 22.06.16 at 11:16, <george.dunlap@citrix.com> wrote:
>>> On 22/06/16 07:39, Jan Beulich wrote:
>>>>>>> On 21.06.16 at 16:38, <george.dunlap@citrix.com> wrote:
>>>>> On 21/06/16 10:47, Jan Beulich wrote:
>>>>>>>>>> And then - didn't we mean to disable that part of XenGT during
>>>>>>>>>> migration, i.e. temporarily accept the higher performance
>>>>>>>>>> overhead without the p2m_ioreq_server entries? In which case
>>>>>>>>>> flipping everything back to p2m_ram_rw after (completed or
>>>>>>>>>> canceled) migration would be exactly what we want. The (new
>>>>>>>>>> or previous) ioreq server should attach only afterwards, and
>>>>>>>>>> can then freely re-establish any p2m_ioreq_server entries it
>>>>>>>>>> deems necessary.
>>>>>>>>>>
>>>>>>>>> Well, I agree this part of XenGT should be disabled during migration.
>>>>>>>>> But in such
>>>>>>>>> case I think it's device model's job to trigger the p2m type
>>>>>>>>> flipping(i.e. by calling
>>>>>>>>> HVMOP_set_mem_type).
>>>>>>>> I agree - this would seem to be the simpler model here, despite (as
>>>>>>>> George validly says) the more consistent model would be for the
>>>>>>>> hypervisor to do the cleanup. Such cleanup would imo be reasonable
>>>>>>>> only if there was an easy way for the hypervisor to enumerate all
>>>>>>>> p2m_ioreq_server pages.
>>>>>>> Well, for me, the "easy way" means we should avoid traversing the whole ept
>>>>>>> paging structure all at once, right?
>>>>>> Yes.
>>>>> Does calling p2m_change_entry_type_global() not satisfy this requirement?
>>>> Not really - that addresses the "low overhead" aspect, but not the
>>>> "enumerate all such entries" one.
>>> I'm sorry, I think I'm missing something here.  What do we need the
>>> enumeration for?
>> We'd need that if we were to do the cleanup in the hypervisor (as
>> we can't rely on all p2m entry re-calculation to have happened by
>> the time a new ioreq server registers for the type).
> So you're afraid of this sequence of events?
> 1) Server A de-registered, triggering a ioreq_server -> ram_rw type change
> 2) gfn N is marked as misconfigured
> 3) Server B registers and marks gfn N as ioreq_server
> 4) When N is accessed, the misconfiguration is resolved incorrectly to
> ram_rw
>
> But that can't happen, because misconfigured entries are resolved before
> setting a p2m entry; so at step 3, gfn N will be first set to
> (non-misconfigured) ram_rw, then changed to (non-misconfigured)
> ioreq_server.
>
> Or is there another sequence of events that I'm missing?

Thanks for your reply, George. :)
If no log dirty is triggered during this process, your sequence is correct.
However, if log dirty is triggered, we'll met problems. I have described 
this
in previous mails :

http://lists.xenproject.org/archives/html/xen-devel/2016-06/msg02426.html
on Jun 20

and

http://lists.xenproject.org/archives/html/xen-devel/2016-06/msg02575.html
on Jun 21

>>>>> Well I had in principle already agreed to letting this be the interface
>>>>> on the previous round of patches; we're having this discussion because
>>>>> you (Jan) asked about what happens if an ioreq server is de-registered
>>>>> while there are still outstanding p2m types. :-)
>>>> Indeed. Yet so far I understood you didn't like de-registration to
>>>> both not do the cleanup itself and fail if there are outstanding
>>>> entries.
>>> No, I think regarding deregistering while there were outstanding
>>> entries, I said the opposite -- that there's no point in failing the
>>> de-registration, because a poorly-behaved ioreq server may just ignore
>>> the error code and exit anyway.  Although, thinking on it again, I
>>> suppose that an error code would allow a buggy ioreq server to know that
>>> it had screwed up somewhere.
>> Not exactly, I think: The failed de-registration ought to lead to failure
>> of an attempt to register another ioreq server (or the same one again),
>> which should make the issue quickly noticable.
> Hmm... yes, the more I think about it the more it seems like allowing
> p2m entries from a previous ioreq server to be already set when there's
> a new ioreq server registration is digging a hole for future people to
> fall into.  Paul and Yu Zhang are the most likely people to fall into
> that hole, so I haven't been arguing strenuously so far against it, but
> given that I'm not yet convinced that fixing it is that difficult, at

Taking the live migration into consideration, I admit I still have not 
found any light-
weighted solution to fix this. :)

Thanks
Yu

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