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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 22/06/16 10:29, Jan Beulich wrote:
>>>> On 22.06.16 at 11:16, <george.dunlap@xxxxxxxxxx> wrote:
>> On 22/06/16 07:39, Jan Beulich wrote:
>>>>>> On 21.06.16 at 16:38, <george.dunlap@xxxxxxxxxx> 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?

>>>> 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
very least I would strongly recommend them to reconsider.

 -George

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel