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 20/06/16 12:28, Yu Zhang wrote:
> 
> 
> On 6/20/2016 6:55 PM, Jan Beulich wrote:
>>>>> On 20.06.16 at 12:32, <george.dunlap@xxxxxxxxxx> wrote:
>>> On 20/06/16 11:25, Jan Beulich wrote:
>>>>>>> On 20.06.16 at 12:10, <george.dunlap@xxxxxxxxxx> wrote:
>>>>> On 20/06/16 10:03, Yu Zhang wrote:
>>>>>> However, there are conflicts if we take live migration  into account,
>>>>>> i.e. if the live migration is
>>>>>> triggered by the user(unintentionally maybe) during the gpu emulation
>>>>>> process, resolve_misconfig()
>>>>>> will set all the outstanding p2m_ioreq_server entries to
>>>>>> p2m_log_dirty,
>>>>>> which is not what we expected,
>>>>>> because our intention is to only reset the outdated p2m_ioreq_server
>>>>>> entries back to p2m_ram_rw.
>>>>> Well the real problem in the situation you describe is that a second
>>>>> "lazy" p2m_change_entry_type_global() operation is starting before the
>>>>> first one is finished.  All that's needed to resolve the situation is
>>>>> that if you get a second p2m_change_entry_type_global() operation
>>>>> while
>>>>> there are outstanding entries from the first type change, you have to
>>>>> finish the first operation (i.e., go "eagerly" find all the
>>>>> misconfigured entries and change them to the new type) before starting
>>>>> the second one.
>>>> Eager resolution of outstanding entries can't be the solution here, I
>>>> think, as that would - afaict - be as time consuming as doing the type
>>>> change synchronously right away.
>>> But isn't it the case that p2m_change_entry_type_global() is only
>>> implemented for EPT?
>> Also for NPT, we're using a similar model in p2m-pt.c (see e.g. the
>> uses of RECALC_FLAGS - we're utilizing the _PAGE_USER set
>> unconditionally leads to NPF). And since shadow sits on top of
>> p2m-pt, that should be covered too.
>>
>>>   So we've been doing the slow method for both
>>> shadow and AMD HAP (whatever it's called these days) since the
>>> beginning.  And in any case we'd only have to go for the "slow" case in
>>> circumstances where the 2nd type change happened before the first one
>>> had completed.
>> We can't even tell when one have fully finished.
> 
> I agree, we have no idea if the previous type change is completely done.
> Besides, IIUC, the p2m_change_entry_type_gobal() is not a quite slow
> method, because it does
> not invalidate all the paging structure entries at once, it just writes
> the upper level ones, others
> are updated in resolve_misconfig().
> 
>>
>>>>   p2m_change_entry_type_global(),
>>>> at least right now, can be invoked freely without prior type changes
>>>> having fully propagated. The logic resolving mis-configured entries
>>>> simply needs to be able to know the correct new type. I can't see
>>>> why this logic shouldn't therefore be extensible to this new type
>>>> which can be in flight - after we ought to have a way to know what
>>>> type a particular GFN is supposed to be?
>>> Actually, come to think of it -- since the first type change is meant to
>>> convert all ioreq_server -> ram_rw, and the second is meant to change
>>> all ram_rw -> logdirty,  is there any case in which we *wouldn't* want
>>> the resulting type to be logdirty?  Isn't that exactly what we'd get if
>>> we'd done both operations synchronously?
>> I think Yu's concern is for pages which did not get converted back?
>> Or on the restore side? Otherwise - "yes" to both of your questions.
>>
> 
> Yes. My concern is that resolve_misconfig() can not easily be extended
> to differentiate the
> p2m_ioreq_server entries which need to be reset and the normal
> p2m_ioreq_server entries.

Under what circumstance should resolve_misconfig() change a
misconfigured entry into a p2m_ioreq_server entry?

 -George

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