Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions

[David Hildenbrand <david@redhat.com>]

>> Same applies to all other kinds of operations (splitting, punching out,
>> ...) as you also mentioned.
> 
> One question from a QEMU newbie though: why do you put
> kvm_ioctl_inhibit_begin()/kvm_ioctl_inhibit_end() to kvm_region_resize()
> only and not taking it all the way up to
> memory_region_transaction_begin()/memory_region_transaction_end() to
> support atomicity for all kinds of updates right away?

The clean way to implement it for 
memory_region_transaction_begin()/memory_region_transaction_end() is by 
implementing
->begin()
->commit()
callbacks for the KVM MemoryListener, and doing it in there, in KVM code.


Now, I wasn't sure how this might affect real-time workloads, where you 
really don't want to kick CPUs out of KVM. You can make a lot of 
operations without requiring this handling like

1. Adding regions (memory hotplug)
2. Removing regions (memory hotunplug)
3. Enabling/disabling dirty logging

Resize/split(/move/...) are the problematic operations where we would 
need that handling. Modifying the size/location of existing slots.

One way to tackle it would be to "sense" upfront if such "modifying" 
operations will be required, communicating that via "->begin()", and 
letting the KVM notifier decide based on that information whether to get 
everything out of KVM. Sounds feasible.

> 
> The second question is whether you plan to sumbit this any time soon)

Once we have an agreement on how to proceed, either I can try to 
dedicate some time, or if you have some time, you can pickup my work and 
make it also handle the other problematic cases (as discussed e.g., ^).

-- 
Thanks,

David / dhildenb