Re: [PATCH 5/5] x86/xen: Don't register PV spinlock IPI when it isn't going to be used

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Mon, 2021-01-04 at 18:44 -0500, boris.ostrovsky@xxxxxxxxxx wrote:
> On 1/4/21 6:19 PM, David Woodhouse wrote:
> > On Mon, 2021-01-04 at 18:04 -0500, boris.ostrovsky@xxxxxxxxxx
> > wrote:
> > > Why not xen_hvm_cpu_die too? common_cpu_die() sounds like something
> > > we should do, and the other three we call there will be nops.
> > 
> > native_cpu_die() calls that, and isn't that the function that gets
> > installed if we don't install our own?
> 
> 
> True.
> 
> 
> Still, a Xen guest should call Xen-specific cpu_die() routine if
> possible. Especially since (now) other cpu (i.e. non-IPI) ops will
> call Xen versions.

But as you said, the other three things that xen_hvm_cpu_die() does are
all no-ops (or at least we'd have to make them no-ops; I haven't
checked). I don't see the point in calling it, only for it to do
nothing.

> 
> > 
> > > >         smp_ops.send_call_func_ipi = xen_smp_send_call_function_ipi;
> > > >         smp_ops.send_call_func_single_ipi = 
> > > > xen_smp_send_call_function_single_ipi;
> > > > -       smp_ops.smp_prepare_boot_cpu = xen_hvm_smp_prepare_boot_cpu;
> > > > -       smp_ops.smp_cpus_done = xen_smp_cpus_done;
> > > >  }
> > > > 
> > > > > Also, for the spinlock changes specifically --- I wonder whether it
> > > > > would be better to reverse initial value of xen_pvspin and set it to
> > > > > 'true' only if initialization succeeds.
> > > > 
> > > > I looked at that but it would need to be tristate, since the
> > > > 'xen_nopvspin' command line option clears it from its default of being
> > > > enabled.
> > > 
> > > Ah, right. How about setting nopvspin in xen_parse_nopvspin()?
> > 
> > That would make the xen_nopvspin command line option disable PV
> > spinlocks even under KVM.
> 
> 
> xen_nopvspin is deprecated and nopvspin is recommended anyway so I think it 
> should be OK, no?

They do different things. If someone has an image (which might run on
both Xen and KVM) which has xen_nopvspin, we don't suddenly want that
to start disabling PV spinlocks on KVM too.

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