> From: Jason Gunthorpe <jgg@xxxxxxxxxx> > Sent: Friday, May 6, 2022 7:46 PM > > On Fri, May 06, 2022 at 03:51:40AM +0000, Tian, Kevin wrote: > > > From: Jason Gunthorpe <jgg@xxxxxxxxxx> > > > Sent: Thursday, May 5, 2022 10:08 PM > > > > > > On Thu, May 05, 2022 at 07:40:37AM +0000, Tian, Kevin wrote: > > > > > > > In concept this is an iommu property instead of a domain property. > > > > > > Not really, domains shouldn't be changing behaviors once they are > > > created. If a domain supports dirty tracking and I attach a new device > > > then it still must support dirty tracking. > > > > That sort of suggests that userspace should specify whether a domain > > supports dirty tracking when it's created. But how does userspace > > know that it should create the domain in this way in the first place? > > live migration is triggered on demand and it may not happen in the > > lifetime of a VM. > > The best you could do is to look at the devices being plugged in at VM > startup, and if they all support live migration then request dirty > tracking, otherwise don't. Yes, this is how a device capability can help. > > However, tt costs nothing to have dirty tracking as long as all iommus > support it in the system - which seems to be the normal case today. > > We should just always turn it on at this point. Then still need a way to report " all iommus support it in the system" to userspace since many old systems don't support it at all. If we all agree that a device capability flag would be helpful on this front (like you also said below), probably can start building the initial skeleton with that in mind? > > > and if the user always creates domain to allow dirty tracking by default, > > how does it know a failed attach is due to missing dirty tracking support > > by the IOMMU and then creates another domain which disables dirty > > tracking and retry-attach again? > > The automatic logic is complicated for sure, if you had a device flag > it would have to figure it out that way > Yes. That is the model in my mind. Thanks Kevin
