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 6/20/2016 9:13 PM, George Dunlap wrote:
> 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@citrix.com> wrote:
>>>> On 20/06/16 11:25, Jan Beulich wrote:
>>>>>>>> On 20.06.16 at 12:10, <george.dunlap@citrix.com> 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 because our intention is to only reset the outdated p2m_ioreq_server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?

Oh, I did not mean that. Routine resolve_misconfig() shall not change
any entry into a p2m_ioreq_server type. I hoped this routine could be
changed to reset outdated p2m_ioreq_server entries(by "outdated" I
refer to the entries which are no longer tracked by an ioreq server but
remain as p2m_ioreq_server) back to p2m_ram_rw type.

Later I realized that we may also change the normal p2m_ioreq_server
entries(by "nomal" I mean the gfns which are in emulation process) if
live migration is triggered during emulation process. And it's hard to
distinguish the outdated p2m_ioreq_server entries and the normal ones.

Thanks
Yu

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