linux-kernel - Re: [PATCH v3 4/4] KVM: arm64: Clear active

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Date:   Tue, 3 Aug 2021 16:30:37 +0100
From:   Will Deacon <will@...nel.org>
To:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "maz@...nel.org" <maz@...nel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "james.morse@....com" <james.morse@....com>,
        "julien.thierry.kdev@...il.com" <julien.thierry.kdev@...il.com>,
        "suzuki.poulose@....com" <suzuki.poulose@....com>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "Alexandru.Elisei@....com" <Alexandru.Elisei@....com>,
        "qperret@...gle.com" <qperret@...gle.com>,
        Linuxarm <linuxarm@...wei.com>
Subject: Re: [PATCH v3 4/4] KVM: arm64: Clear active_vmids on vCPU schedule
 out

On Tue, Aug 03, 2021 at 12:55:25PM +0000, Shameerali Kolothum Thodi wrote:
> > > 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.

Why didn't you say so?!

> > 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

I think we have to be really careful not to run into the "suspended
animation" problem described in ae120d9edfe9 ("ARM: 7767/1: let the ASID
allocator handle suspended animation") if we go down this road.

Maybe something along the lines of:

ROLLOVER

  * Take lock
  * Inc generation
    => This will force everybody down the slow path
  * Record active VMIDs
  * Broadcast TLBI
    => Only active VMIDs can be dirty
    => Reserve active VMIDs and mark as allocated

VCPU SCHED IN

  * Set active VMID
  * Check generation
  * If mismatch then:
        * Take lock
        * Try to match a reserved VMID
        * If no reserved VMID, allocate new

VCPU SCHED OUT

  * Clear active VMID

but I'm not daft enough to think I got it right first time. I think it
needs both implementing *and* modelling in TLA+ before we merge it!

Will