Re: [PATCH RFC 4/4] kvm: Implement atomic memory region resizes via region_resize()

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On 06/03/20 11:20, David Hildenbrand wrote:
> Yeah, rwlocks are not optimal and I am still looking for better
> alternatives (suggestions welcome :) ). Using RCU might not work,
> because the rcu_read region might be too big (esp. while in KVM_RUN).
> 
> I had a prototype which used a bunch of atomics + qemu_cond_wait. But it
> was quite elaborate and buggy.
> 
> (I assume only going into KVM_RUN is really affected, and I do wonder if
> it will be noticeable at all. Doing an ioctl is always already an
> expensive operation.)
> 
> I can look into per-cpu locks instead of the rwlock.

Assuming we're only talking about CPU ioctls (seems like a good
approximation) maybe you could use start_exclusive/end_exclusive?  The
current_cpu->in_exclusive_context assignments can be made conditional on
"if (current_cpu)".

However that means you have to drop the BQL, see
process_queued_cpu_work.  It may be a problem.


Paolo




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