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 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?

>> 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.  But either way, from the "robustness"
perspective, the result would almost certainly be a dangling ioreq
server registration *in addition* to the dangling p2m entries; so the
difference is just an interface tweak to aid in debugging, not worth
insisting on given the required work.

 -George


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