On Fri, Oct 20, 2023 at 11:58:58AM +0000, Huang, Kai wrote:
> On Tue, 2023-10-10 at 10:24 +0000, Huang, Kai wrote:
> > > /* Physical address of the Multiprocessor Wakeup Structure mailbox */
> > > @@ -74,6 +75,9 @@ int __init acpi_parse_mp_wake(union acpi_subtable_headers *header,
> > >
> > >
> > > acpi_mp_wake_mailbox_paddr = mp_wake->base_address;
> > >
> > >
> > > + /* Disable CPU onlining/offlining */
> > > + cpu_hotplug_not_supported();
> > > +
> >
> > Both onlining/offlining are prevented, or just offlining?
> >
> > The previous patch says:
> >
> > It does not prevent the initial bring up of the CPU, but it stops
> > subsequent offlining.
> >
> > And ...
> >
> > [...]
> >
> >
> > > --- a/kernel/cpu.c
> > > +++ b/kernel/cpu.c
> > > @@ -1522,7 +1522,7 @@ static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target)
> > > * If the platform does not support hotplug, report it explicitly to
> > > * differentiate it from a transient offlining failure.
> > > */
> > > - if (cc_platform_has(CC_ATTR_HOTPLUG_DISABLED) || !cpu_hotplug_supported)
> > > + if (!cpu_hotplug_supported)
> > > return -EOPNOTSUPP;
> > > if (cpu_hotplug_disabled)
> > > return -EBUSY;
> >
> > ... here cpu_down_maps_locked() only prevents offlining if I am reading
> > correctly.
> >
> > Also, can we rename cpu_hotplug_supported to cpu_offline_supported to match the
> > behaviour better?
> >
> > Anyway, isn't it a little bit odd to have:
> >
> > if (!cpu_hotplug_supported)
> > return -EOPNOTSUPP;
> > if (cpu_hotplug_disabled)
> > return -EBUSY;
> >
> > ?
>
> I probably have missed something important, but I don't quite understand what's
> the reason to have the CC_ATTR_HOTPLUG_DISABLED at the beginning, and now
> replace it with cpu_hotplug_not_supported()?
CC_ATTR_HOTPLUG_DISABLED was a mistake. And now obvious when we only need
to disable offlining dynamically, based on supported MADT MP WP version.
> From the changelog:
>
> Currently hotplug prevented based on the confidential computing
> attribute which is set for Intel TDX. But TDX is not the only possible
> user of the wake up method.
>
> CC_ATTR_HOTPLUG_DISABLED is only used by TDX guest, but MADT can be used by non-
> TDX guest too.
>
> Anyway, if the purpose is just to prevent CPU from going offline, can this be
> done by registering a cpuhp callback?
>
> static int madt_wakeup_offline_cpu(unsigned int cpu)
> {
> return -EOPNOTSUPP;
> }
>
> ...
>
> err = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "madt-wakeup",
> NULL, madt_wakeup_offline_cpu);
> if (err) {
> pr_err("Register CPU hotplug callback failed.\n");
> /* BUG() ??? */
> }
>
> This doesn't pollute the common CPU hotplug code, thus to me looks more clear?
Thomas seems fine with cpu_hotplug_disable_offlining().
--
Kiryl Shutsemau / Kirill A. Shutemov
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