Re: [PATCH v3 06/22] KVM: arm64: Simplify the CPUHP logic

Isaku Yamahata
2022-09-07T15:12:22Z

On Mon, Sep 05, 2022 at 01:39:20PM +0100,
Marc Zyngier <maz@kernel.org> wrote:

> On 2022-09-05 10:29, Marc Zyngier wrote:
> > On Mon, 05 Sep 2022 08:05:09 +0100,
> > Yuan Yao <yuan.yao@linux.intel.com> wrote:
> > >
> > > On Thu, Sep 01, 2022 at 07:17:41PM -0700, isaku.yamahata@intel.com
> > > wrote:
> > > > From: Marc Zyngier <maz@kernel.org>
> > > >
> > > > For a number of historical reasons, the KVM/arm64 hotplug setup is pretty
> > > > complicated, and we have two extra CPUHP notifiers for vGIC and timers.
> > > >
> > > > It looks pretty pointless, and gets in the way of further changes.
> > > > So let's just expose some helpers that can be called from the core
> > > > CPUHP callback, and get rid of everything else.
> > > >
> > > > This gives us the opportunity to drop a useless notifier entry,
> > > > as well as tidy-up the timer enable/disable, which was a bit odd.
> > > >
> > > > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > > > Signed-off-by: Chao Gao <chao.gao@intel.com>
> > > > Reviewed-by: Oliver Upton <oupton@google.com>
> > > > Link: https://lore.kernel.org/r/20220216031528.92558-5-chao.gao@intel.com
> > > > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > > > ---
> > > > arch/arm64/kvm/arch_timer.c | 27 ++++++++++-----------------
> > > > arch/arm64/kvm/arm.c | 4 ++++
> > > > arch/arm64/kvm/vgic/vgic-init.c | 19 ++-----------------
> > > > include/kvm/arm_arch_timer.h | 4 ++++
> > > > include/kvm/arm_vgic.h | 4 ++++
> > > > include/linux/cpuhotplug.h | 3 ---
> > > > 6 files changed, 24 insertions(+), 37 deletions(-)
> > > >
> > > > diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
> > > > index bb24a76b4224..33fca1a691a5 100644
> > > > --- a/arch/arm64/kvm/arch_timer.c
> > > > +++ b/arch/arm64/kvm/arch_timer.c
> > > > @@ -811,10 +811,18 @@ void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu)
> > > > ptimer->host_timer_irq_flags = host_ptimer_irq_flags;
> > > > }
> > > >
> > > > -static void kvm_timer_init_interrupt(void *info)
> > > > +void kvm_timer_cpu_up(void)
> > > > {
> > > > enable_percpu_irq(host_vtimer_irq, host_vtimer_irq_flags);
> > > > - enable_percpu_irq(host_ptimer_irq, host_ptimer_irq_flags);
> > > > + if (host_ptimer_irq)
> > > > + enable_percpu_irq(host_ptimer_irq, host_ptimer_irq_flags);
> > > > +}
> > > > +
> > > > +void kvm_timer_cpu_down(void)
> > > > +{
> > > > + disable_percpu_irq(host_vtimer_irq);
> > > > + if (host_ptimer_irq)
> > > > + disable_percpu_irq(host_ptimer_irq);
> > > > }
> > >
> > > Should "host_vtimer_irq" be checked yet as host_ptimer_irq ?
> >
> > No, because although the ptimer interrupt is optional (on older
> > systems, we fully emulate that timer, including the interrupt), the
> > vtimer interrupt is always present and can be used unconditionally.
> >
> > > Because
> > > the host_{v,p}timer_irq is set in same function kvm_irq_init() which
> > > called AFTER the on_each_cpu(_kvm_arch_hardware_enable, NULL, 1) from
> > > init_subsystems():
> > >
> > > kvm_init()
> > > kvm_arch_init()
> > > init_subsystems()
> > > on_each_cpu(_kvm_arch_hardware_enable, NULL, 1);
> > > kvm_timer_hyp_init()
> > > kvm_irq_init()
> > > host_vtimer_irq = info->virtual_irq;
> > > host_ptimer_irq = info->physical_irq;
> > > hardware_enable_all()
> >
> > This, however, is a very nice catch. I doubt this results in anything
> > really bad (the interrupt enable will fail as the interrupt number
> > is 0, and the disable will also fail due to no prior enable), but
> > that's extremely ugly anyway.
> >
> > The best course of action AFAICS is to differentiate between the
> > arm64-specific initialisation (which is a one-off) and the runtime
> > stuff. Something like the hack below, that I haven't tested yet:
> >
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 32c1022eb4b3..65d03c28f32a 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -1671,23 +1671,27 @@ static void _kvm_arch_hardware_enable(void
> > *discard)
> > {
> > if (!__this_cpu_read(kvm_arm_hardware_enabled)) {
> > cpu_hyp_reinit();
> > - kvm_vgic_cpu_up();
> > - kvm_timer_cpu_up();
> > __this_cpu_write(kvm_arm_hardware_enabled, 1);
> > }
> > }
> >
> > int kvm_arch_hardware_enable(void)
> > {
> > + int was_enabled = __this_cpu_read(kvm_arm_hardware_enabled);
> > +
> > _kvm_arch_hardware_enable(NULL);
> > +
> > + if (!was_enabled) {
> > + kvm_vgic_cpu_up();
> > + kvm_timer_cpu_up();
> > + }
> > +
> > return 0;
> > }
> >
> > static void _kvm_arch_hardware_disable(void *discard)
> > {
> > if (__this_cpu_read(kvm_arm_hardware_enabled)) {
> > - kvm_timer_cpu_down();
> > - kvm_vgic_cpu_down();
> > cpu_hyp_reset();
> > __this_cpu_write(kvm_arm_hardware_enabled, 0);
> > }
> > @@ -1695,6 +1699,11 @@ static void _kvm_arch_hardware_disable(void
> > *discard)
> >
> > void kvm_arch_hardware_disable(void)
> > {
> > + if (__this_cpu_read(kvm_arm_hardware_enabled)) {
> > + kvm_timer_cpu_down();
> > + kvm_vgic_cpu_down();
> > + }
> > +
> > if (!is_protected_kvm_enabled())
> > _kvm_arch_hardware_disable(NULL);
> > }
>
> OK, this seems to work here, at least based on a sample of 2
> systems, bringing CPUs up and down whist a VM is pinned to
> these CPUs.
>
> Isaku, can you please squash this into the original patch
> and drop Oliver's Reviewed-by: tag, as this significantly
> changes the logic?
>
> Alternatively, I can repost this patch as a standalone change.

I'll do with the next respin. Anyway feel free to go before me.
--
Isaku Yamahata <isaku.yamahata@gmail.com>