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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.06.16 at 11:47, <george.dunlap@xxxxxxxxxx> wrote:
> 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?

1) Server A marks GFN Y as ioreq_server
2) Server A de-registered, triggering a ioreq_server -> ram_rw type
   change
3) Server B registers and gfn Y still didn't become ram_rw again (as
   the misconfiguration didn't trickle down the tree far enough)

Jan


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