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

[Paolo Bonzini]

On 06/03/20 15:30, David Hildenbrand wrote:
>> 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.
>>
> Yeah, start_exclusive() is expected to be called without the BQL,
> otherwise the other CPUs would not be able to make progress and can
> eventually be "caught".
> 
> It's essentially the same reason why I can't use high-level
> pause_all_vcpus()/resume_all_vcpus(). Will drop the BQL which is very
> bad for resizing code.

But any other synchronization primitive that you do which blocks all
vCPUs will have the same issue, otherwise you get a deadlock.

Paolo