RE: [PATCH v3 4/4] KVM: arm64: Clear active_vmids on vCPU schedule out

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Will Deacon [mailto:will@xxxxxxxxxx]
> Sent: 03 August 2021 12:41
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; kvmarm@xxxxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; maz@xxxxxxxxxx; catalin.marinas@xxxxxxx;
> james.morse@xxxxxxx; julien.thierry.kdev@xxxxxxxxx;
> suzuki.poulose@xxxxxxx; jean-philippe@xxxxxxxxxx;
> Alexandru.Elisei@xxxxxxx; qperret@xxxxxxxxxx; Linuxarm
> <linuxarm@xxxxxxxxxx>
> Subject: Re: [PATCH v3 4/4] KVM: arm64: Clear active_vmids on vCPU
> schedule out
> 
> On Thu, Jul 29, 2021 at 11:40:09AM +0100, Shameer Kolothum wrote:
> > Like ASID allocator, we copy the active_vmids into the
> > reserved_vmids on a rollover. But it's unlikely that
> > every CPU will have a vCPU as current task and we may
> > end up unnecessarily reserving the VMID space.
> >
> > Hence, clear active_vmids when scheduling out a vCPU.
> >
> > Suggested-by: Will Deacon <will@xxxxxxxxxx>
> > Signed-off-by: Shameer Kolothum
> <shameerali.kolothum.thodi@xxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/kvm_host.h | 1 +
> >  arch/arm64/kvm/arm.c              | 1 +
> >  arch/arm64/kvm/vmid.c             | 6 ++++++
> >  3 files changed, 8 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h
> b/arch/arm64/include/asm/kvm_host.h
> > index bb993bce1363..d93141cb8d16 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -687,6 +687,7 @@ extern unsigned int kvm_arm_vmid_bits;
> >  int kvm_arm_vmid_alloc_init(void);
> >  void kvm_arm_vmid_alloc_free(void);
> >  void kvm_arm_vmid_update(struct kvm_vmid *kvm_vmid);
> > +void kvm_arm_vmid_clear_active(void);
> >
> >  static inline void kvm_arm_pvtime_vcpu_init(struct kvm_vcpu_arch
> *vcpu_arch)
> >  {
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 077e55a511a9..b134a1b89c84 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -435,6 +435,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> >  	kvm_timer_vcpu_put(vcpu);
> >  	kvm_vgic_put(vcpu);
> >  	kvm_vcpu_pmu_restore_host(vcpu);
> > +	kvm_arm_vmid_clear_active();
> >
> >  	vcpu->cpu = -1;
> >  }
> > diff --git a/arch/arm64/kvm/vmid.c b/arch/arm64/kvm/vmid.c
> > index 5584e84aed95..5fd51f5445c1 100644
> > --- a/arch/arm64/kvm/vmid.c
> > +++ b/arch/arm64/kvm/vmid.c
> > @@ -116,6 +116,12 @@ static u64 new_vmid(struct kvm_vmid
> *kvm_vmid)
> >  	return idx2vmid(vmid) | generation;
> >  }
> >
> > +/* Call with preemption disabled */
> > +void kvm_arm_vmid_clear_active(void)
> > +{
> > +	atomic64_set(this_cpu_ptr(&active_vmids), 0);
> > +}
> 
> I think this is very broken, as it will force everybody to take the
> slow-path when they see an active_vmid of 0.

Yes. I have seen that happening in my test setup.

> It also doesn't solve the issue I mentioned before, as an active_vmid of 0
> means that the reserved vmid is preserved.
> 
> Needs more thought...

How about we clear all the active_vmids in kvm_arch_free_vm() if it
matches the kvm_vmid->id ? But we may have to hold the lock 
there.

Thanks,
Shameer