linux-kernel - Re: [PATCH v4 22/32] KVM: s390: pci: routines for (dis)associating zPCI devices with a KVM

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Date:   Tue, 15 Mar 2022 12:39:13 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.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, joro@...tes.org, will@...nel.org,
        pbonzini@...hat.com, corbet@....net, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 22/32] KVM: s390: pci: routines for (dis)associating
 zPCI devices with a KVM

On 3/14/22 5:46 PM, Jason Gunthorpe wrote:
> On Mon, Mar 14, 2022 at 03:44:41PM -0400, Matthew Rosato wrote:
>> +int kvm_s390_pci_zpci_start(struct kvm *kvm, struct zpci_dev *zdev)
>> +{
>> +	struct vfio_device *vdev;
>> +	struct pci_dev *pdev;
>> +	int rc;
>> +
>> +	rc = kvm_s390_pci_dev_open(zdev);
>> +	if (rc)
>> +		return rc;
>> +
>> +	pdev = pci_get_slot(zdev->zbus->bus, zdev->devfn);
>> +	if (!pdev) {
>> +		rc = -ENODEV;
>> +		goto exit_err;
>> +	}
>> +
>> +	vdev = get_vdev(&pdev->dev);
>> +	if (!vdev) {
>> +		pci_dev_put(pdev);
>> +		rc = -ENODEV;
>> +		goto exit_err;
>> +	}
>> +
>> +	zdev->kzdev->nb.notifier_call = kvm_s390_pci_group_notifier;
>> +
>> +	/*
>> +	 * At this point, a KVM should already be associated with this device,
>> +	 * so registering the notifier now should immediately trigger the
>> +	 * event.  We also want to know if the KVM association is later removed
>> +	 * to ensure proper cleanup happens.
>> +	 */
>> +	rc = register_notifier(vdev->dev, &zdev->kzdev->nb);
>> +
>> +	put_vdev(vdev);
>> +	pci_dev_put(pdev);
>> +
>> +	/* Make sure the registered KVM matches the KVM issuing the ioctl */
>> +	if (rc || zdev->kzdev->kvm != kvm) {
>> +		rc = -ENODEV;
>> +		goto exit_err;
>> +	}
>> +
>> +	/* Must support KVM-managed IOMMU to proceed */
>> +	if (IS_ENABLED(CONFIG_S390_KVM_IOMMU))
>> +		rc = zpci_iommu_attach_kvm(zdev, kvm);
>> +	else
>> +		rc = -EINVAL;
> 
> This seems like kind of a strange API, shouldn't kvm be getting a
> reference on the underlying iommu_domain and then calling into it to
> get the mapping table instead of pushing KVM specific logic into the
> iommu driver?
> 
> I would be nice if all the special kvm stuff could more isolated in
> kvm code.
> 
> I'm still a little unclear about why this is so complicated - can't
> you get the iommu_domain from the group FD directly in KVM code as
> power does?

Yeah, I think I could do something like that using the vfio group fd 
like power does.

Providing a reference to the kvm itself inside iommu was being used for 
the pin/unpin operations, which would not be necessary if we switched to 
the 1st layer iommu pinning all of guest memory.