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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