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

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On 22.01.2018 13:09, Janosch Frank wrote:
> On 22.01.2018 12:56, 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.
>>>
>>> Signed-off-by: Janosch Frank <frankja@xxxxxxxxxxxxxxxxxx>
>>> ---
>>>  arch/s390/include/asm/gmap.h    |  3 ++
>>>  arch/s390/include/asm/pgtable.h |  7 +++-
>>>  arch/s390/mm/gmap.c             | 92 ++++++++++++++++++++++++++++++++++++-----
>>>  arch/s390/mm/pgtable.c          |  4 ++
>>>  4 files changed, 94 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/arch/s390/include/asm/gmap.h b/arch/s390/include/asm/gmap.h
>>> index c1bc563..21bb658 100644
>>> --- a/arch/s390/include/asm/gmap.h
>>> +++ b/arch/s390/include/asm/gmap.h
>>> @@ -13,6 +13,9 @@
>>>  #define GMAP_NOTIFY_SHADOW	0x2
>>>  #define GMAP_NOTIFY_MPROT	0x1
>>>  
>>> +/* Status bits in the gmap segment entry. */
>>> +#define _SEGMENT_ENTRY_GMAP_IN		0x0001	/* invalidation notify bit */
>>> +
>>
>> _SEGMENT_ENTRY_READ -> 0x0001
>>
>> Is it even okay to reuse that bit?
>>
> 
> It's in the GMAP segment entry, not in the process' segment entry.
> That's why we throw away all software bits from the process entry when
> linking into the gmap table.
> 

Ah right, we only share page tables but not complete segments. Thanks.

-- 

Thanks,

David / dhildenb
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