On Fri, Aug 27, 2021, Mathieu Desnoyers wrote: > > So there are effectively three reasons we want a delay: > > > > 1. To allow sched_setaffinity() to coincide with ioctl(KVM_RUN) before KVM can > > enter the guest so that the guest doesn't need an arch-specific VM-Exit source. > > > > 2. To let ioctl(KVM_RUN) make its way back to the test before the next round > > of migration. > > > > 3. To ensure the read-side can make forward progress, e.g. if sched_getcpu() > > involves a syscall. > > > > > > After looking at KVM for arm64 and s390, #1 is a bit tenuous because x86 is the > > only arch that currently uses xfer_to_guest_mode_work(), i.e. the test could be > > tweaked to be overtly x86-specific. But since a delay is needed for #2 and #3, > > I'd prefer to rely on it for #1 as well in the hopes that this test provides > > coverage for arm64 and/or s390 if they're ever converted to use the common > > xfer_to_guest_mode_work(). > > Now that we have this understanding of why we need the delay, it would be good to > write this down in a comment within the test. Ya, I'll get a new version out next week. > Does it reproduce if we randomize the delay to have it picked randomly from 0us > to 100us (with 1us step) ? It would remove a lot of the needs for arch-specific > magic delay value. My less-than-scientific testing shows that it can reproduce at delays up to ~500us, but above ~10us the reproducibility starts to drop. The bug still reproduces reliably, it just takes more iterations, and obviously the test runs a bit slower. Any objection to using a 1-10us delay, e.g. a simple usleep((i % 10) + 1)?
