Re: [PATCH v3 1/4] KVM: arm64: Introduce a new VMID allocator for KVM

[Will Deacon]

On Thu, Jul 29, 2021 at 11:40:06AM +0100, Shameer Kolothum wrote:
> A new VMID allocator for arm64 KVM use. This is based on
> arm64 ASID allocator algorithm.
> 
> One major deviation from the ASID allocator is the way we
> flush the context. Unlike ASID allocator, we expect less
> frequent rollover in the case of VMIDs. Hence, instead of
> marking the CPU as flush_pending and issuing a local context
> invalidation on the next context switch, we broadcast TLB
> flush + I-cache invalidation over the inner shareable domain
> on rollover.
> 
> Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@xxxxxxxxxx>
> ---

[...]

> +void kvm_arm_vmid_update(struct kvm_vmid *kvm_vmid)
> +{
> +	unsigned long flags;
> +	unsigned int cpu;
> +	u64 vmid, old_active_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))
> +		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);

new_vmid() can just set kvm_vmid->id directly

> +	}
> +
> +	cpu = smp_processor_id();

Why?

Will