Re: [PATCH 0/4] KVM: API to block and resume all running vcpus in a vm

[Paolo Bonzini]

On 10/25/22 00:45, Sean Christopherson wrote:
> > Yes that helps and should be part of the cover letter for the next iterations.
> But that doesn't explain why KVM needs to get involved, it only explains why QEMU
> can't use its existing pause_all_vcpus().  I do not understand why this is a
> problem QEMU needs KVM's help to solve.

I agree that it's not KVM's problem that QEMU cannot use 
pause_all_vcpus().  Having a ioctl in KVM, rather than coding the same 
in QEMU, is *mostly* a matter of programmer and computer efficiency 
because the code is pretty simple.

That said, I believe the limited memslot API makes it more than just a 
QEMU problem.  Because KVM_GET_DIRTY_LOG cannot be combined atomically 
with KVM_SET_USER_MEMORY_REGION(MR_DELETE), any VMM that uses dirty-log 
regions while the VM is running is liable to losing the dirty status of 
some pages.  That's also a reason to provide this API in KVM.

Paolo