Re: [PATCH 02/35] KVM: s390/interrupt: do not pin adapter interrupt pages

[David Hildenbrand <david@redhat.com>]

> Am 10.02.2020 um 19:41 schrieb Christian Borntraeger <borntraeger@de.ibm.com>:
> 
> 
> 
>> On 10.02.20 13:26, David Hildenbrand wrote:
>>> On 07.02.20 12:39, Christian Borntraeger wrote:
>>> From: Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
>>> 
>>> The adapter interrupt page containing the indicator bits is currently
>>> pinned. That means that a guest with many devices can pin a lot of
>>> memory pages in the host. This also complicates the reference tracking
>>> which is needed for memory management handling of protected virtual
>>> machines.
>>> We can reuse the pte notifiers to "cache" the page without pinning it.
>>> 
>>> Signed-off-by: Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
>>> Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
>>> [borntraeger@de.ibm.com: patch merging, splitting, fixing]
>>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>> ---
>> 
>> So, instead of pinning explicitly, look up the page address, cache it,
>> and glue its lifetime to the gmap table entry. When that entry is
>> changed, invalidate the cached page. On re-access, look up the page
>> again and register the gmap notifier for the table entry again.
> 
> I think I might want to split this into two parts.
> part 1: a naive approach that always does get_user_pages_remote/put_page
> part 2: do the complex caching
> 
> Ulrich mentioned that this actually could make the map/unmap a no-op as we
> have the address and bit already in the irq route. In the end this might be
> as fast as todays pinning as we replace a list walk with a page table walk. 
> Plus it would simplify the code. Will have a look if that is the case.

If we could simplify that heavily, that would be awesome!