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

Marc Zyngier
2022-09-05T12:43:18Z

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.

Thanks,

M.
--
Jazz is not dead. It just smells funny...