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

[David Hildenbrand <david@redhat.com>]

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