On Tue, 2 Feb 2021 08:28:52 +0100 Erik Skultety <eskul...@redhat.com> wrote:
> On Mon, Feb 01, 2021 at 04:38:56PM -0700, Alex Williamson wrote: > > On Mon, 1 Feb 2021 16:57:44 -0600 > > Jonathon Jongsma <jjong...@redhat.com> wrote: > > > > > On Mon, 1 Feb 2021 11:33:08 +0100 > > > Erik Skultety <eskul...@redhat.com> wrote: > > > > > > > On Mon, Feb 01, 2021 at 09:48:11AM +0000, Daniel P. Berrangé > > > > wrote: > > > > > On Mon, Feb 01, 2021 at 10:45:43AM +0100, Erik Skultety > > > > > wrote: > > > > > > On Mon, Feb 01, 2021 at 09:42:32AM +0000, Daniel P. Berrangé > > > > > > wrote: > > > > > > > On Fri, Jan 29, 2021 at 05:34:36PM -0600, Jonathon Jongsma > > > > > > > wrote: > > > > > > > > On Thu, 7 Jan 2021 17:43:54 +0100 > > > > > > > > Erik Skultety <eskul...@redhat.com> wrote: > > > > > > > > > > > > > > > > > > Tested with v6.10.0-283-g1948d4e61e. > > > > > > > > > > > > > > > > > > > > 1.Can define/start/destroy mdev device successfully; > > > > > > > > > > > > > > > > > > > > 2.'virsh nodedev-list' has no '--active' option, > > > > > > > > > > which is inconsistent with the description in the > > > > > > > > > > patch： # virsh nodedev-list --active > > > > > > > > > > error: command 'nodedev-list' doesn't support option > > > > > > > > > > --active > > > > > > > > > > > > > > > > > > > > 3.virsh client hang when trying to destroy a mdev > > > > > > > > > > device which is using by a vm, and after that all > > > > > > > > > > 'virsh nodev*' cmds will hang. If restarting > > > > > > > > > > llibvirtd after that, libvirtd will hang. > > > > > > > > > > > > > > > > > > It hangs because underneath a write to the 'remove' > > > > > > > > > sysfs attribute is now blocking for some reason and > > > > > > > > > since we're relying on mdevctl to do it for us, hence > > > > > > > > > "it hangs". I'm not trying to make an excuse, it's > > > > > > > > > plain wrong. I'd love to rely on such a basic > > > > > > > > > functionality, but it looks like we'll have to go > > > > > > > > > with a extremely ugly workaround and try to get the > > > > > > > > > list of active domains from the nodedev driver and > > > > > > > > > see whether any of them has the device assigned > > > > > > > > > before we try to destroy the mdev via the nodedev > > > > > > > > > driver. > > > > > > > > > > > > > > > > So, I've been trying to figure out a way to do this, > > > > > > > > but as far as I know, there's no way to get a list of > > > > > > > > active domains from within the nodedev driver, and I > > > > > > > > can't think of any better ways to handle it. Any ideas? > > > > > > > > > > > > > > > > > > > > > > Correct, the nodedev driver isn't permitted to talk to > > > > > > > any of the virt drivers. > > > > > > > > > > > > Oh, not even via secondary connection? What makes nodedev so > > > > > > special, since we can open a secondary connection from e.g. > > > > > > the storage driver? > > > > > > > > > > It is technically possible, but it should never be done, > > > > > because it introduces a bi-directional dependancy between the > > > > > daemons which introduces the danger of deadlocking them. None > > > > > of the secondary drivers should connect to the hypervisor > > > > > drivers. > > > > > > > Is there anything in sysfs which reports whether the > > > > > > > device is in use ? > > > > > > > > > > > > Nothing that I know of, the way it used to work was that you > > > > > > tried to write to sysfs and kernel returned a write error > > > > > > with "device in use" or something like that, but that has > > > > > > changed since :(. > > > > > > > > Without having tried this and since mdevctl is just a Bash > > > > script, can we bypass mdevctl on destroys a little bit by > > > > constructing the path to the sysfs attribute ourselves and > > > > perform a non-blocking write of zero bytes to see if we get an > > > > error? If so, we'll skip mdevctl and report an error. If we > > > > don't, we'll invoke mdevctl to remove the device in order to > > > > remain consistent. Would that be an acceptable workaround > > > > (provided it would work)? > > > > > > As far as I can tell, this doesn't work. According to my > > > tests, attempting to write zero bytes to $(mdev_sysfs_path)/remove > > > doesn't result in an error if the mdev is in use by a vm. It just > > > "successfully" writes zero bytes. Adding Alex to cc in case he > > > has an idea for a workaround here. > > > > [Cc +Connie] > > > > I'm not really sure why mdevs are unique here. When we write to > > remove, the first step is to release the device from the driver, so > > it's really the same as an unbind for a vfio-pci device. PCI > > drivers cannot return an error, an unbind is handled not as a > > request, but a directive, so when the device is in use we block > > until the unbind can complete. With vfio-pci (and added upstream > > to the mdev core - depending on vendor support), the driver remove > > callback triggers a virtual interrupt to the user asking to > > cooperatively return the device (triggering a hot-unplug in QEMU). > > Has this really worked so well in vfio-pci that we've forgotten > > that an unbind can block there too or are we better about tracking > > something with PCI devices vs mdevs? > > Does any of the current vendor guest drivers for mdev support unplug? > While I'm not trying to argue that unpluging a vfio-pci cannot block, > it just works seamlessly in majority of cases nowadays, but I guess > we were in the same situation with PCI assignment in the past? > > The whole point here is IMO about a massive inconvenience for a > library consumer to be blocked on an operation and not knowing why, > whereas when you return an instant error saying why the operation > cannot be completed right now that opens the door for a necessary > adjustment in their usage of the library. > > > > > On idea for a solution would be that vfio only allows a single open > > of a group at a time, so if libvirt were to open the group it could > > know that it's unused. If you can manage to close the group once > > you've already triggered the remove/unbind, then I'd think the > > completion of the write would be deterministic. If the group is in > > use elsewhere, the open should get back an -EBUSY and you probably > > ought to be careful > > Honestly, ^this seems like a fairly straightforward workaround to me. > > Erik Yes, thanks. This seems pretty clean and simple to to implement, and I can't really think of any significant downsides. Jonathon > > > about removing/unbinding it anyway. It might be possible to > > implement this in mdevctl too, ie. redirect /dev/null to group file > > and fork, fork the echo 1 > remove, kill the redirect, return a > > device in use error if the initial redirect fails. Thanks, > > > > Alex
