Ankur Arora <ankur.a.arora@xxxxxxxxxx> writes: > A KVM host (or another hypervisor) might advertise paravirtualized > features and optimization hints (ex KVM_HINTS_REALTIME) which might > become stale over the lifetime of the guest. For instance, the > host might go from being undersubscribed to being oversubscribed > (or the other way round) and it would make sense for the guest > switch pv-ops based on that. If your host changes his advertised behaviour then you want to fix the host setup or find a competent admin. > This lockorture splat that I saw on the guest while testing this is > indicative of the problem: > > [ 1136.461522] watchdog: BUG: soft lockup - CPU#8 stuck for 22s! [lock_torture_wr:12865] > [ 1136.461542] CPU: 8 PID: 12865 Comm: lock_torture_wr Tainted: G W L 5.4.0-rc7+ #77 > [ 1136.461546] RIP: 0010:native_queued_spin_lock_slowpath+0x15/0x220 > > (Caused by an oversubscribed host but using mismatched native pv_lock_ops > on the gues.) And this illustrates what? The fact that you used a misconfigured setup. > This series addresses the problem by doing paravirt switching at > runtime. You're not addressing the problem. Your fixing the symptom, which is wrong to begin with. > The alternative use-case is a runtime version of apply_alternatives() > (not posted with this patch-set) that can be used for some safe subset > of X86_FEATUREs. This could be useful in conjunction with the ongoing > late microcode loading work that Mihai Carabas and others have been > working on. This has been discussed to death before and there is no safe subset as long as this hasn't been resolved: https://lore.kernel.org/lkml/alpine.DEB.2.21.1909062237580.1902@xxxxxxxxxxxxxxxxxxxxxxx/ Thanks, tglx
