Re: [PATCH v3 1/1] vfio: remove VFIO_GROUP_NOTIFY_SET_KVM

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 1/5/23 7:32 PM, Jason Gunthorpe wrote:
> On Thu, Jan 05, 2023 at 07:16:37PM -0500, Matthew Rosato wrote:
> 
>> Yeah, this is also what I was thinking, replace the direct
>> kvm_put_kvm calls with, say, schedule_delayed_work in each driver,
>> where the delayed task just does the kvm_put_kvm (along with a brief
>> comment explaining why we handle the put asynchronously).
> 
> Don't put that in every driver, do something like mmput_async() where
> the core code has all of this.
> 

If the core vfio code were to add logic to invoke kvm_put_kvm and kvm_get_kvm, won't this introduce a vfio dependency on kvm?  If I recall, we have the drivers handling the kvm reference today in order to avoid that..

>> Other than that..  The goal of this patch originally was to get the
>> kvm reference at first open_device and release it with the very last
>> close_device, so the only other option I could think of would be to
>> take the responsibility back from the vfio drivers and do the
>> kvm_get_kvm and kvm_put_kvm directly in vfio_main after dropping the
>> (but that would result in some ugly symbol linkage and would acquire
>> kvm references that a driver maybe does not care about so I don't
>> really like that idea)
> 
> And we still have the deadlock problem anyhow..

Looks like I never finished my sentence here -- I meant call kvm_put_kvm directly in vfio_main after dropping the group lock (e.g. when we set device->kvm = NULL;). But I think we'd still have the kvm dependency issue