On Thu, Mar 10, 2016 at 07:33:05PM +1100, Paul Mackerras wrote: > On Mon, Mar 07, 2016 at 05:00:14PM +1100, David Gibson wrote: > > On Mon, Mar 07, 2016 at 02:41:11PM +1100, Alexey Kardashevskiy wrote: > > > VFIO on sPAPR already implements guest memory pre-registration > > > when the entire guest RAM gets pinned. This can be used to translate > > > the physical address of a guest page containing the TCE list > > > from H_PUT_TCE_INDIRECT. > > > > > > This makes use of the pre-registrered memory API to access TCE list > > > pages in order to avoid unnecessary locking on the KVM memory > > > reverse map. > > > > > > Signed-off-by: Alexey Kardashevskiy <a...@ozlabs.ru> > > > > Ok.. so, what's the benefit of not having to lock the rmap? > > It's not primarily about locking or not locking the rmap. The point > is that when memory is pre-registered, we know that all of guest > memory is pinned and we have a flat array mapping GPA to HPA. It's > simpler and quicker to index into that array (even with looking up the > kernel page tables in vmalloc_to_phys) than it is to find the memslot, > lock the rmap entry, look up the user page tables, and unlock the rmap > entry. We were only locking the rmap entry to stop the page being > unmapped and reallocated to something else, but if memory is > pre-registered, it's all pinned, so it can't be reallocated.
Ok, that makes sense.
Alexey, can you fold some of that rationale into the commit message?
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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