linux-kernel - Re: [PATCH v6 15/21] KVM: s390: pci: add routines to start/stop interpretive execution

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Date:   Wed, 27 Apr 2022 12:14:00 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, alex.williamson@...hat.com,
        cohuck@...hat.com, schnelle@...ux.ibm.com, farman@...ux.ibm.com,
        pmorel@...ux.ibm.com, borntraeger@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, gerald.schaefer@...ux.ibm.com,
        agordeev@...ux.ibm.com, svens@...ux.ibm.com, frankja@...ux.ibm.com,
        david@...hat.com, imbrenda@...ux.ibm.com, vneethv@...ux.ibm.com,
        oberpar@...ux.ibm.com, freude@...ux.ibm.com, thuth@...hat.com,
        pasic@...ux.ibm.com, pbonzini@...hat.com, corbet@....net,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v6 15/21] KVM: s390: pci: add routines to start/stop
 interpretive execution

On Tue, Apr 26, 2022 at 04:08:36PM -0400, Matthew Rosato wrote:

> +int kvm_s390_pci_register_kvm(struct zpci_dev *zdev, struct kvm *kvm)
> +{
> +	if (!zdev)
> +		return 0;
> +
> +	/*
> +	 * Register device with this KVM (or remove the KVM association if 0).
> +	 * If interpetation facilities are available, enable them and let
> +	 * userspace indicate whether or not they will be used (specify SHM bit
> +	 * to disable).
> +	 */
> +	if (kvm)
> +		return register_kvm(zdev, kvm);
> +	else
> +		return unregister_kvm(zdev);
> +}
> +EXPORT_SYMBOL_GPL(kvm_s390_pci_register_kvm);

I think it is cleaner to expose both the register/unregister APIs and
not multiplex them like this

> +void kvm_s390_pci_clear_list(struct kvm *kvm)
> +{
> +	struct kvm_zdev *tmp, *kzdev;
> +	LIST_HEAD(remove);
> +
> +	spin_lock(&kvm->arch.kzdev_list_lock);
> +	list_for_each_entry_safe(kzdev, tmp, &kvm->arch.kzdev_list, entry)
> +		list_move_tail(&kzdev->entry, &remove);
> +	spin_unlock(&kvm->arch.kzdev_list_lock);
> +
> +	list_for_each_entry_safe(kzdev, tmp, &remove, entry)
> +		unregister_kvm(kzdev->zdev);

Hum, I wonder if this is a mistake in kvm:

static void kvm_destroy_vm(struct kvm *kvm)
{
[..]
	kvm_arch_destroy_vm(kvm);
	kvm_destroy_devices(kvm);

kvm_destroy_devices() triggers the VFIO notifier with NULL. Indeed for
correctness I would expect the VFIO users to have been notified to
release the kvm before the kvm object becomes partially destroyed?

Maybe you should investigate re-ordering this at the KVM side and just
WARN_ON(!list_empty) in the arch code?

(vfio has this odd usage model where it should use the kvm pointer
without taking a ref on it so long as the unregister hasn't been
called)

If you keep it like this then the locking in register/unregister looks
not broad enough and has to cover the zdev->kzdev also.

Overall I think it is OK designed like this, aside from the ugly
symbol_get in vfio which I hope you can resolve.

Jason