On 04.09.24 22:43, David Hildenbrand wrote:
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?
Ah, I remember that kvm_get_free_slot() is also rather inefficient
because we don't "close holes" when removing slots. So we would have to
walk up to the "highest slot ever used". Let me take a look at the patch.
--
Cheers,
David / dhildenb
