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

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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?

-- 

Thanks,

David / dhildenb
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