On Fri, 3 Feb 2023 13:32:09 +0000 "Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote: > > From: Tian, Kevin <kevin.tian@xxxxxxxxx> > > Sent: Friday, February 3, 2023 10:00 AM > > > > > From: Alex Williamson <alex.williamson@xxxxxxxxxx> > > > Sent: Friday, February 3, 2023 7:13 AM > > > > > > On Thu, 2 Feb 2023 23:04:10 +0000 > > > "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote: > > > > > > > > From: Alex Williamson <alex.williamson@xxxxxxxxxx> > > > > > Sent: Friday, February 3, 2023 3:42 AM > > > > > > > > > > > > > > > LGTM. I'm not sure moving the functions to vfio_main really buys us > > > > > anything since we're making so much use of group fields. The cdev > > > > > approach will necessarily be different, so the bulk of the get code will > > > > > likely need to move back to group.c anyway. > > > > > > > > > > > > > well my last comment was based on Matthew's v2 where the get code > > > > gets a kvm passed in instead of implicitly retrieving group ref_lock > > > > internally. In that case the get/put helpers only contain device logic > > > > thus fit in vfio_main.c. > > > > > > > > with v3 then they have to be in group.c since we don't want to use > > > > group fields in vfio_main.c. > > > > > > > > but I still think v2 of the helpers is slightly better. The only difference > > > > between cdev and group when handling this race is using different > > > > ref_lock. the symbol get/put part is exactly same. So even if we > > > > merge v3 like this, very likely Yi has to change it back to v2 style > > > > to share the get/put helpers while just leaving the ref_lock part > > > > handled differently between the two path. > > > > > > I'm not really a fan of the asymmetry of the v2 version where the get > > > helper needs to be called under the new kvm_ref_lock, but the put > > > helper does not. Having the get helper handle that makes the caller > > > much cleaner. Thanks, > > > > > > > What about passing the lock pointer into the helper? it's still slightly > > asymmetry as the put helper doesn't carry the lock pointer but it > > could also be interpreted as if the pointer has been saved in the get > > then if it needs to be referenced by the put there is no need to pass > > it in again. > > For cdev, I may modify vfio_device_get_kvm_safe() to accept > struct kvm and let its caller hold a kvm_ref_lock (field within > struct vfio_device_file). Meanwhile, the group path holds > the group->kvm_ref_lock before invoking vfio_device_get_kvm_safe(). > vfio_device_get_kvm_safe() just includes the symbol get/put and > the device->kvm and put_kvm set. Sounds a lot like v2 :-\ I'd look more towards group and cdev specific helpers that handle the locking so that the callers aren't exposed to the asymmetry of get vs put, and reduce a new _vfio_device_get_kvm_safe() in common code that only does the symbol work. Thanks, Alex
