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


> -----Original Message-----
> From: Shameerali Kolothum Thodi
> Sent: 11 August 2021 09:48
> To: 'Will Deacon' <will@kernel.org>
> Cc: linux-arm-kernel@lists.infradead.org; kvmarm@lists.cs.columbia.edu;
> linux-kernel@vger.kernel.org; maz@kernel.org; catalin.marinas@arm.com;
> james.morse@arm.com; julien.thierry.kdev@gmail.com;
> suzuki.poulose@arm.com; jean-philippe@linaro.org;
> Alexandru.Elisei@arm.com; qperret@google.com; Linuxarm
> <linuxarm@huawei.com>
> Subject: RE: [PATCH v3 4/4] KVM: arm64: Clear active_vmids on vCPU
> schedule out
>
> Hi Will,
>
> > -----Original Message-----
> > From: Will Deacon [mailto:will@kernel.org]
> > Sent: 03 August 2021 16:31
> > To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
> > Cc: linux-arm-kernel@lists.infradead.org; kvmarm@lists.cs.columbia.edu;
> > linux-kernel@vger.kernel.org; maz@kernel.org; catalin.marinas@arm.com;
> > james.morse@arm.com; julien.thierry.kdev@gmail.com;
> > suzuki.poulose@arm.com; jean-philippe@linaro.org;
> > Alexandru.Elisei@arm.com; qperret@google.com; Linuxarm
> > <linuxarm@huawei.com>
> > Subject: Re: [PATCH v3 4/4] KVM: arm64: Clear active_vmids on vCPU
> > schedule out
>
> [...]
>
> > 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!
>
> I attempted to implement the above algo as below. It seems to be
> working in both 16-bit vmid and 4-bit vmid test setup.

It is not :(. I did an extended, overnight test run and it fails.
It looks to me in my below implementation there is no synchronization
on setting the active VMID and a concurrent rollover. I will have another go.

Thanks,
Shameer

Though I am
> not quite sure this Is exactly what you had in mind above and covers
> all corner cases.
>
> Please take a look and let me know.
> (The diff below is against this v3 series)
>
> Thanks,
> Shameer
>
> --->8<----
>
> --- a/arch/arm64/kvm/vmid.c
> +++ b/arch/arm64/kvm/vmid.c
> @@ -43,7 +43,7 @@ static void flush_context(void)
> bitmap_clear(vmid_map, 0, NUM_USER_VMIDS);
>
> for_each_possible_cpu(cpu) {
> - vmid = atomic64_xchg_relaxed(&per_cpu(active_vmids,
> cpu), 0);
> + vmid = atomic64_read(&per_cpu(active_vmids, cpu));
>
> /* Preserve reserved VMID */
> if (vmid == 0)
> @@ -125,32 +125,17 @@ void kvm_arm_vmid_clear_active(void)
> void kvm_arm_vmid_update(struct kvm_vmid *kvm_vmid)
> {
> unsigned long flags;
> - u64 vmid, old_active_vmid;
> + u64 vmid;
>
> vmid = atomic64_read(&kvm_vmid->id);
> -
> - /*
> - * Please refer comments in check_and_switch_context() in
> - * arch/arm64/mm/context.c.
> - */
> - old_active_vmid = atomic64_read(this_cpu_ptr(&active_vmids));
> - if (old_active_vmid && vmid_gen_match(vmid) &&
> - atomic64_cmpxchg_relaxed(this_cpu_ptr(&active_vmids),
> - old_active_vmid, vmid))
> + if (vmid_gen_match(vmid)) {
> + atomic64_set(this_cpu_ptr(&active_vmids), vmid);
> return;
> -
> - raw_spin_lock_irqsave(&cpu_vmid_lock, flags);
> -
> - /* Check that our VMID belongs to the current generation. */
> - vmid = atomic64_read(&kvm_vmid->id);
> - if (!vmid_gen_match(vmid)) {
> - vmid = new_vmid(kvm_vmid);
> - atomic64_set(&kvm_vmid->id, vmid);
> }
>
> -
> + raw_spin_lock_irqsave(&cpu_vmid_lock, flags);
> + vmid = new_vmid(kvm_vmid);
> + atomic64_set(&kvm_vmid->id, vmid);
> atomic64_set(this_cpu_ptr(&active_vmids), vmid);
> raw_spin_unlock_irqrestore(&cpu_vmid_lock, flags);
> }
> --->8<----
>
>
>

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