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

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Will Deacon [mailto:will@xxxxxxxxxx]
> Sent: 03 August 2021 12:39
> 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 1/4] KVM: arm64: Introduce a new VMID allocator for
> KVM
> 
> 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

Ok.
> 
> > +	}
> > +
> > +	cpu = smp_processor_id();
> 
> Why?

Left over from previous one. Forgot to remove
as we don't have the tlb_flush_pending check anymore.

Thanks,
Shameer

> 
> Will