Re: [RFC PATCH 00/26] Runtime paravirt patching

[Ankur Arora]

On 2020-04-08 7:12 a.m., Thomas Gleixner wrote:
> 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. I was thinking of fairly limited subset: ex re-evaluate
X86_FEATURE_ALWAYS to make sure static_cpu_has() reflects reality
but I guess that has second order effects here.

Ankur

> Thanks,
>
>          tglx