Re: [libvirt PATCH 00/17] qemu: Implement external limit manager feature

On Mon, 2021-03-08 at 10:54 +0000, Daniel P. Berrangé wrote: > On Fri, Mar 05, 2021 at 08:13:47PM +0100, Andrea Bolognani wrote: > > This feature has been requested by KubeVirt developers and will make > > it possible for them to make some VFIO-related features, such as > > migration and hotplug, work correctly. > > > > https://bugzilla.redhat.com/show_bug.cgi?id=1916346 > > > > The first part of the series, especially the first 9 patches, is > > preparation work: it addresses a few annoying issues with our APIs > > that deal with process limits, and makes them all nice, consistent > > and easy to reason about while moving policy code from the generic > > code to the QEMU driver where it belongs. > > IIUC, the code was already supposed to attempt to set the limits > and gracefully carry on if it was unable todo so. Can you outline > the actual problem being solved, as wading through the bug comments > to understand the precise detail of the problem is not very clear. Not being able to set the memory locking limit shouldn't be a soft failure, because that might cause QEMU to start up apparently fine and then error out later on during the lifetime of the VM, which is much worse than not starting up at all. We're actually doing this correctly for the most part, which is why hotplug requests for VFIO devices in KubeVirt result in errors under certain conditions.

The reason why VFIO device assignment is currently not completely broken in KubeVirt is that, when the QEMU process is initially started, we set the memory locking limit after fork(), so we can do that using setrlimit() which doesn't require additional capabilities, but in the hotplug scenario libvirtd needs to change the limits of a different process: in that case we are forced to use prlimit(), which fails due to the lack of CAP_SYS_RESOURCE.