Re: [RFC/PATCH v2 03/22] s390/mm: add gmap PMD invalidation notification

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On 22.01.2018 14:29, David Hildenbrand wrote:
> On 22.01.2018 14:13, Janosch Frank wrote:
>> On 22.01.2018 12:46, David Hildenbrand wrote:
>>> On 13.12.2017 13:53, Janosch Frank wrote:
>>>> For later migration of huge pages we want to write-protect guest
>>>> PMDs. While doing this, we have to make absolutely sure, that the
>>>> guest's lowcore is always accessible when the VCPU is running. With
>>>> PTEs, this is solved by marking the PGSTEs of the lowcore pages with
>>>> the invalidation notification bit and kicking the guest out of the SIE
>>>> via a notifier function if we need to invalidate such a page.
>>>>
>>>> With PMDs we do not have PGSTEs or some other bits we could use in the
>>>> host PMD. Instead we pick one of the free bits in the gmap PMD. Every
>>>> time a host pmd will be invalidated, we will check if the respective
>>>> gmap PMD has the bit set and in that case fire up the notifier.
>>>>
>>>> In the first step we only support setting the invalidation bit, but we
>>>> do not support restricting access of guest pmds. It will follow
>>>> shortly.
>>>
>>> I am wondering if we could avoid having invalidation bits on PMDs
>>> completely by always splitting up a PMD huge page into PTEs.
>>>
>>> I assume this would make the code easier - as we need split up of PMDs
>>> either way when protecting for the shadow gmap.
>>>
>>> This would imply that also our notification handler only has to be
>>> called for 4k pages, which also makes that part easier.
>>
>> Except for 1MB shadowed segments which still need an invalidation handler.
> 
> But that doesn't go via gmap_protect_range() / gmap_call_notifier() if I
> am not mistaking.

Yes, my point was that we'd still need pmdp_notify.

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