On Tue, 8 Aug 2017 16:06:43 +0530
Gautham R Shenoy <ego@xxxxxxxxxxxxxxxxxx> wrote:
> Hi Nicholas,
>
> On Sun, Aug 06, 2017 at 03:02:38AM +1000, Nicholas Piggin wrote:
> > POWER9 CPUs have independent MMU contexts per thread so KVM
> > does not have to bring sibling threads into real-mode when
> > switching MMU mode to guest. This can simplify POWER9 sleep/wake
> > paths and avoids hwsyncs.
> >
> > @@ -444,6 +439,7 @@ ALT_FTR_SECTION_END_IFSET(CPU_FTR_ARCH_300)
> > beq 1f
> > b kvm_start_guest
> > 1:
> > +END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_300)
>
> This would be 7 nops on power9. Should we move this to a different
> function and do a bl to that?
Yes that's a good idea.
> > +static void kvmppc_release_hwthread_secondary(int cpu)
> > +{
> > + struct paca_struct *tpaca;
> > +
> > + if (cpu_has_feature(CPU_FTR_ARCH_300)) {
> > + WARN_ON(1);
> > + return;
> > + }
> > +
> > + tpaca = &paca[cpu];
> > + tpaca->kvm_hstate.hwthread_req = 0;
> > + kvmppc_release_hwthread(cpu);
> > +}
> > +
> > +
>
> Extra blank line not needed.
Sure.
> > @@ -2858,11 +2883,13 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
> >
> > /* Let secondaries go back to the offline loop */
> > for (i = 0; i < controlled_threads; ++i) {
> > - kvmppc_release_hwthread(pcpu + i);
> > if (sip && sip->napped[i])
> > kvmppc_ipi_thread(pcpu + i);
> > cpumask_clear_cpu(pcpu + i, &vc->kvm->arch.cpu_in_guest);
> > }
>
> We are sending an IPI to the thread that has exited the guest and is
> currently napping. The IPI wakes it up so that it can executes
> offline loop. But we haven't released the hwthread yet, which means
> that hwthread_req for this thread is still set.
>
> The thread wakes up from nap, executes the pnv_powersave_wakeup code
> where it can enter kvm_start_guest. Is this a legitimate race or am I
> missing something?
Oh I think it's just a silly mistake in my patch, good catch.
Would moving this loop below the one below solve it? I wasn't
completely happy with uglifying these loops by making the
primary release different than secondary... maybe I will just
move the difference into kvmppc_release_hwthread and which is
less intrusive to callers.
Thanks,
Nick
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