Re: [PATCH 1/5] iommu/virtio: Break out bypass identity support into a global static

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Feb 21, 2025 at 11:35:27AM +0000, Jean-Philippe Brucker wrote:
> > +static int viommu_attach_identity_domain(struct iommu_domain *domain,
> > +					 struct device *dev)
> > +{
> > +	int ret = 0;
> > +	struct virtio_iommu_req_attach req;
> > +	struct viommu_endpoint *vdev = dev_iommu_priv_get(dev);
> > +	struct viommu_domain *vdomain = to_viommu_domain(domain);
> > +
> > +	req = (struct virtio_iommu_req_attach) {
> > +		.head.type	= VIRTIO_IOMMU_T_ATTACH,
> > +		.domain		= cpu_to_le32(vdev->viommu->identity_domain_id),
> > +		.flags          = cpu_to_le32(VIRTIO_IOMMU_ATTACH_F_BYPASS),
> > +	};
> > +
> > +	ret = viommu_send_attach_req(vdev->viommu, dev, &req);
> > +	if (ret)
> > +		return ret;
> > +
> > +	if (vdev->vdomain) {
> > +		vdev->vdomain->nr_endpoints--;
> > +		vdomain->nr_endpoints++;
> > +		vdev->vdomain = vdomain;
> 
> These two need to be unconditional

Woops yes

> > +static struct viommu_domain viommu_identity_domain = {
> > +	.domain = { .type = IOMMU_DOMAIN_IDENTITY,
> > +		    .ops =
> > +			    &(const struct iommu_domain_ops){
> > +				    .attach_dev = viommu_attach_identity_domain,
> > +			    } }
> > +};
> 
> nit: how about
> 
> 	static struct viommu_domain viommu_identity_domain = {
> 		.domain = {
> 			.type = IOMMU_DOMAIN_IDENTITY,
> 			.ops = &(const struct iommu_domain_ops) {
> 				.attach_dev = viommu_attach_identity_domain,
> 			},
> 		},
> 	};

Done

> > +	/* Reserve an ID to use as the bypass domain */
> > +	if (virtio_has_feature(viommu->vdev, VIRTIO_IOMMU_F_BYPASS_CONFIG)) {
> > +		viommu->identity_domain_id = viommu->first_domain;
> > +		viommu->first_domain++;
> > +	} else {
> > +		/*
> > +		 * Assume the VMM is sensible and it either supports bypass on
> > +		 * all instances or no instances.
> > +		 */
> 
> Maybe also a WARN_ON(!viommu_ops.identity_domain) above?

It starts working in the following patch because the core will call
viommu_domain_alloc_identity() that can make a correct
per-device/per-viommu determination of bypass support.

Let me fold this into the next patch to make that clearer:

@@ -998,7 +998,7 @@ static void viommu_get_resv_regions(struct device *dev, struct list_head *head)
        iommu_dma_get_resv_regions(dev, head);
 }
 
-static struct iommu_ops viommu_ops;
+static const struct iommu_ops viommu_ops;
 static struct virtio_driver virtio_iommu_drv;
 
 static int viommu_match_node(struct device *dev, const void *data)
@@ -1086,8 +1086,7 @@ static bool viommu_capable(struct device *dev, enum iommu_cap cap)
        }
 }
 
-static struct iommu_ops viommu_ops = {
-       .identity_domain        = &viommu_identity_domain.domain,
+static const struct iommu_ops viommu_ops = {
        .capable                = viommu_capable,
        .domain_alloc_identity  = viommu_domain_alloc_identity,
        .domain_alloc_paging    = viommu_domain_alloc_paging,
@@ -1216,12 +1215,6 @@ static int viommu_probe(struct virtio_device *vdev)
        if (virtio_has_feature(viommu->vdev, VIRTIO_IOMMU_F_BYPASS_CONFIG)) {
                viommu->identity_domain_id = viommu->first_domain;
                viommu->first_domain++;
-       } else {
-               /*
-                * Assume the VMM is sensible and it either supports bypass on
-                * all instances or no instances.
-                */
-               viommu_ops.identity_domain = NULL;
        }

Jason