Re: [PATCH] KVM: arm64: Fix CPUHP logic for protected KVM

Raghavendra Rao Ananta
2023-07-19T21:16:58Z

On Wed, Jul 19, 2023 at 1:48 PM Oliver Upton <oliver.upton@linux.dev> wrote:
>
> On Wed, Jul 19, 2023 at 05:54:00PM +0000, Raghavendra Rao Ananta wrote:
> > For protected kvm, the CPU hotplug 'down' logic currently brings
> > down the timer and vGIC, essentially disabling interrupts. However,
> > because of how the 'kvm_arm_hardware_enabled' flag is designed, it
> > never re-enables them back on the CPU hotplug 'up' path. Hence,
> > clean up the logic to maintain the CPU hotplug up/down symmetry.
>
> Correct me if I am wrong, but this issue exists outside of cpu hotplug,
> right? init_subsystems() calls _kvm_arch_hardware_enable() on all cores,
> which only sets up the hyp cpu context and not the percpu interrupts.
> Similar issue exists for the cpu that calls do_pkvm_init().
>
Ah, perhaps I looked at the from a different perspective, but this
makes sense too.

> I'll also note kvm_arm_hardware_enabled is deceptively vague, as it only
> keeps track of whether or not the hyp cpu context has been initialized.
> May send a cleanup here in a bit.
>
> Perhaps this for the changelog:
>
> KVM: arm64: Fix hardware enable/disable flows for pKVM
>
> When running in protected mode, the hyp stub is disabled after pKVM is
> initialized, meaning the host cannot enable/disable the hyp at
> runtime. As such, kvm_arm_hardware_enabled is always 1 after
> initialization, and kvm_arch_hardware_enable() never enables the vgic
> maintenance irq or timer irqs.
>
> Unconditionally enable/disable the vgic + timer irqs in the respective
> calls, instead relying on the percpu bookkeeping in the generic code
> to keep track of which cpus have the interrupts unmasked.
>
Sure, we can use this for v2.

Thanks,
Raghavendra
> > Fixes: 466d27e48d7c ("KVM: arm64: Simplify the CPUHP logic")
> > Reported-by: Oliver Upton <oliver.upton@linux.dev>
> > Suggested-by: Oliver Upton <oliver.upton@linux.dev>
> > Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
> > ---
> > arch/arm64/kvm/arm.c | 14 ++++----------
> > 1 file changed, 4 insertions(+), 10 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index c2c14059f6a8..010ebfa69650 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -1867,14 +1867,10 @@ static void _kvm_arch_hardware_enable(void *discard)
> >
> > 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();
> > - }
> > + kvm_vgic_cpu_up();
> > + kvm_timer_cpu_up();
> >
> > return 0;
> > }
> > @@ -1889,10 +1885,8 @@ 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();
> > - }
> > + kvm_timer_cpu_down();
> > + kvm_vgic_cpu_down();
> >
> > if (!is_protected_kvm_enabled())
> > _kvm_arch_hardware_disable(NULL);
> > --
> > 2.41.0.487.g6d72f3e995-goog
> >
>
> --
> Thanks,
> Oliver