On Wed, Jun 15, 2022 at 07:35:00AM +0000, Tian, Kevin wrote: > External email: Use caution opening links or attachments > > > > From: Nicolin Chen <nicolinc@xxxxxxxxxx> > > Sent: Wednesday, June 15, 2022 4:45 AM > > > > Hi Kevin, > > > > On Wed, Jun 08, 2022 at 11:48:27PM +0000, Tian, Kevin wrote: > > > > > > The KVM mechanism for controlling wbinvd is only triggered during > > > > > > kvm_vfio_group_add(), meaning it is a one-shot test done once the > > > > devices > > > > > > are setup. > > > > > > > > > > It's not one-shot. kvm_vfio_update_coherency() is called in both > > > > > group_add() and group_del(). Then the coherency property is > > > > > checked dynamically in wbinvd emulation: > > > > > > > > From the perspective of managing the domains that is still > > > > one-shot. It doesn't get updated when individual devices are > > > > added/removed to domains. > > > > > > It's unchanged per-domain but dynamic per-vm when multiple > > > domains are added/removed (i.e. kvm->arch.noncoherent_dma_count). > > > It's the latter being checked in the kvm. > > > > I am going to send a v2, yet not quite getting the point here. > > Meanwhile, Jason is on leave. > > > > What, in your opinion, would be an accurate description here? > > > > Something like below: > -- > The KVM mechanism for controlling wbinvd is based on OR of > the coherency property of all devices attached to a guest, no matter > those devices are attached to a single domain or multiple domains. > > So, there is no value in trying to push a device that could do enforced > cache coherency to a dedicated domain vs re-using an existing domain > which is non-coherent since KVM won't be able to take advantage of it. > This just wastes domain memory. > > Simplify this code and eliminate the test. This removes the only logic > that needed to have a dummy domain attached prior to searching for a > matching domain and simplifies the next patches. > > It's unclear whether we want to further optimize the Intel driver to > update the domain coherency after a device is detached from it, at > least not before KVM can be verified to handle such dynamics in related > emulation paths (wbinvd, vcpu load, write_cr0, ept, etc.). In reality > we don't see an usage requiring such optimization as the only device > which imposes such non-coherency is Intel GPU which even doesn't > support hotplug/hot remove. Thanks! I just updated that and will send v2.
