RE: [RFC PATCH v2 2/7] KVM: arm64: Introduce support to pin VMIDs

[Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>]

> -----Original Message-----
> From: Oliver Upton <oliver.upton@linux.dev>
> Sent: Monday, June 24, 2024 8:22 PM
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
> Cc: kvmarm@lists.linux.dev; iommu@lists.linux.dev; linux-arm-
> kernel@lists.infradead.org; Linuxarm <linuxarm@huawei.com>;
> kevin.tian@intel.com; jgg@ziepe.ca; alex.williamson@redhat.com;
> maz@kernel.org; will@kernel.org; robin.murphy@arm.com; jean-
> philippe@linaro.org; Jonathan Cameron <jonathan.cameron@huawei.com>
> Subject: Re: [RFC PATCH v2 2/7] KVM: arm64: Introduce support to pin
> VMIDs
> 
> Hi Shameer,
> 
> On Thu, Feb 08, 2024 at 03:18:32PM +0000, Shameer Kolothum wrote:
> 
> [...]
> 
> > +unsigned long kvm_arm_pinned_vmid_get(struct kvm_vmid *kvm_vmid)
> > +{
> > +	unsigned long flags;
> > +	u64 vmid;
> > +
> > +	if (!pinned_vmid_map)
> > +		return 0;
> 
> Like I'd mentioned over in the other thread, returning 0 for error
> conditions is rather confusing unless one is staring at the VMID
> allocator code.

Agree. It gives the impression VMID 0 is valid.

> 
> Can you rework this to return an actual error (e.g. -EINVAL) on failure?

Ok.

> 
> > +	raw_spin_lock_irqsave(&cpu_vmid_lock, flags);
> > +
> > +	vmid = atomic64_read(&kvm_vmid->id);
> > +
> > +	if (refcount_inc_not_zero(&kvm_vmid->pinned))
> > +		goto out_unlock;
> > +
> > +	if (nr_pinned_vmids >= max_pinned_vmids) {
> > +		vmid = 0;
> > +		goto out_unlock;
> > +	}
> 
> You need to decrement the refcount in this error path.

Hmm..do we? We are here means refcount is zero, right?

> 
> > +
> > +	/*
> > +	 * If we went through one or more rollover since that VMID was
> > +	 * used, make sure it is still valid, or generate a new one.
> > +	 */
> > +	if (!vmid_gen_match(vmid))
> > +		vmid = new_vmid(kvm_vmid);
> > +
> > +	nr_pinned_vmids++;
> > +	__set_bit(vmid2idx(vmid), pinned_vmid_map);
> > +	refcount_set(&kvm_vmid->pinned, 1);
> > +
> > +out_unlock:
> > +	raw_spin_unlock_irqrestore(&cpu_vmid_lock, flags);
> > +
> > +	vmid &= ~VMID_MASK;
> > +
> > +	return vmid;
> > +}
> > +
> > +void kvm_arm_pinned_vmid_put(struct kvm_vmid *kvm_vmid)
> > +{
> > +	unsigned long flags;
> > +	u64 vmid = atomic64_read(&kvm_vmid->id);
> > +
> > +	if (!pinned_vmid_map)
> > +		return;
> > +
> > +	raw_spin_lock_irqsave(&cpu_vmid_lock, flags);
> > +
> > +	if (refcount_dec_and_test(&kvm_vmid->pinned)) {
> > +		__clear_bit(vmid2idx(vmid), pinned_vmid_map);
> > +		nr_pinned_vmids--;
> > +	}
> > +
> > +	raw_spin_unlock_irqrestore(&cpu_vmid_lock, flags);
> > +}
> > +
> >  /*
> >   * Initialize the VMID allocator
> >   */
> > @@ -191,10 +263,20 @@ int __init kvm_arm_vmid_alloc_init(void)
> >  	if (!vmid_map)
> >  		return -ENOMEM;
> >
> > +	pinned_vmid_map = bitmap_zalloc(NUM_USER_VMIDS,
> GFP_KERNEL);
> > +	nr_pinned_vmids = 0;
> > +
> > +	/*
> > +	 * Ensure we have at least one emty slot available after rollover
> 
> typo: empty

Thanks,
Shameer