Re: [PATCH 3/4] KVM: Dynamic sized kvm memslots array

[David Hildenbrand <david@redhat.com>]

On 04.09.24 21:16, Peter Xu wrote:
> Zhiyi reported an infinite loop issue in VFIO use case.  The cause of that
> was a separate discussion, however during that I found a regression of
> dirty sync slowness when profiling.
> 
> Each KVMMemoryListerner maintains an array of kvm memslots.  Currently it's
> statically allocated to be the max supported by the kernel.  However after
> Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"),
> the max supported memslots reported now grows to some number large enough
> so that it may not be wise to always statically allocate with the max
> reported.
> 
> What's worse, QEMU kvm code still walks all the allocated memslots entries
> to do any form of lookups.  It can drastically slow down all memslot
> operations because each of such loop can run over 32K times on the new
> kernels.
> 
> Fix this issue by making the memslots to be allocated dynamically.

Wouldn't it be sufficient to limit the walk to the actually used slots?

I know, the large allocation might sound scary at first, but memory 
overcommit+populate-on-demand should handle that, assuming nobody 
touches the yet-unused slots.

-- 
Cheers,

David / dhildenb