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

[Isaku Yamahata]

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

> On 2022-09-05 10:29, Marc Zyngier wrote:
> > On Mon, 05 Sep 2022 08:05:09 +0100,
> > Yuan Yao <yuan.yao@xxxxxxxxxxxxxxx> wrote:
> > > 
> > > On Thu, Sep 01, 2022 at 07:17:41PM -0700, isaku.yamahata@xxxxxxxxx
> > > wrote:
> > > > From: Marc Zyngier <maz@xxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxx>
> > > > Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> > > > Reviewed-by: Oliver Upton <oupton@xxxxxxxxxx>
> > > > Link: https://lore.kernel.org/r/20220216031528.92558-5-chao.gao@xxxxxxxxx
> > > > Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > > > ---
> > > >  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@xxxxxxxxx>