[PATCH 0/2] Faster MMU lookups for Book3s v3

Alexander Graf agraf at suse.de
Thu Jul 1 22:28:06 EST 2010

Avi Kivity wrote:
> On 07/01/2010 01:00 PM, Alexander Graf wrote:
>> But doesn't that mean that you still need to loop through all the hvas
>> that you want to invalidate?
> It does.
>>   Wouldn't it speed up dirty bitmap flushing
>> a lot if we'd just have a simple linked list of all sPTEs belonging to
>> that memslot?
> The complexity is O(pages_in_slot) + O(sptes_for_slot).
> Usually, every page is mapped at least once, so sptes_for_slot
> dominates.  Even when it isn't so, iterating the rmap base pointers is
> very fast since they are linear in memory, while sptes are scattered
> around, causing cache misses.

Why would pages be mapped often? Don't you use lazy spte updates?

> Another consideration is that on x86, an spte occupies just 64 bits
> (for the hardware pte); if there are multiple sptes per page (rare on
> modern hardware), there is also extra memory for rmap chains;
> sometimes we also allocate 64 bits for the gfn.  Having an extra
> linked list would require more memory to be allocated and maintained.

Hrm. I was thinking of not having an rmap but only using the chain. The
only slots that would require such a chain would be the ones with dirty
bitmapping enabled, so no penalty for normal RAM (unless you use kemari
or live migration of course).

But then again I probably do need an rmap for the mmu_notifier magic,
right? But I'd rather prefer to have that code path be slow and the
dirty bitmap invalidation fast than the other way around. Swapping is
slow either way.


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