On Thu, 20 Apr 2023 03:13:02 +0100,
Reiji Watanabe <reijiw@xxxxxxxxxx> wrote:
>
> Hi Marc,
>
> On Wed, Apr 19, 2023 at 08:12:45AM +0100, Marc Zyngier wrote:
> > On Wed, 19 Apr 2023 03:18:51 +0100,
> > Reiji Watanabe <reijiw@xxxxxxxxxx> wrote:
> > > kvm_arch_vcpu_ioctl_vcpu_init() doesn't acquire mp_state_lock
> > > when setting the mp_state to KVM_MP_STATE_RUNNABLE. Fix the
> > > code to acquire the lock.
> > >
> > > Signed-off-by: Reiji Watanabe <reijiw@xxxxxxxxxx>
> > > ---
> > > arch/arm64/kvm/arm.c | 5 ++++-
> > > 1 file changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > > index fbafcbbcc463..388aa4f18f21 100644
> > > --- a/arch/arm64/kvm/arm.c
> > > +++ b/arch/arm64/kvm/arm.c
> > > @@ -1244,8 +1244,11 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
> > > */
> > > if (test_bit(KVM_ARM_VCPU_POWER_OFF, vcpu->arch.features))
> > > kvm_arm_vcpu_power_off(vcpu);
> > > - else
> > > + else {
> > > + spin_lock(&vcpu->arch.mp_state_lock);
> > > WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_RUNNABLE);
> > > + spin_unlock(&vcpu->arch.mp_state_lock);
> > > + }
> > >
> > > return 0;
> > > }
> >
> > I'm not entirely convinced that this fixes anything. What does the
> > lock hazard against given that the write is atomic? But maybe a
>
> It appears that kvm_psci_vcpu_on() expects the vCPU's mp_state
> to not be changed by holding the lock. Although I don't think this
> code practically causes any real issues now, I am a little concerned
> about leaving one instance that updates mpstate without acquiring the
> lock, in terms of future maintenance, as holding the lock won't prevent
> mp_state from being updated.
>
> What do you think ?
Right, fair enough. It is probably better to take the lock and not
have to think of this sort of things... I'm becoming more lazy by the
minute!
>
> > slightly more readable of this would be to expand the critical section
> > this way:
> >
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 4ec888fdd4f7..bb21d0c25de7 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -1246,11 +1246,15 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu,
> > /*
> > * Handle the "start in power-off" case.
> > */
> > + spin_lock(&vcpu->arch.mp_state_lock);
> > +
> > if (test_bit(KVM_ARM_VCPU_POWER_OFF, vcpu->arch.features))
> > - kvm_arm_vcpu_power_off(vcpu);
> > + __kvm_arm_vcpu_power_off(vcpu);
> > else
> > WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_RUNNABLE);
> >
> > + spin_unlock(&vcpu->arch.mp_state_lock);
> > +
> > return 0;
> > }
> >
> > Thoughts?
>
> Yes, it looks better!
Cool. I've applied this change to your patch, applied the series to
the lock inversion branch, and remerged the branch in -next.
We're getting there! ;-)
M.
--
Without deviation from the norm, progress is not possible.
