[RFC] iommu/vt-d: Group and domain relationship

[James Sewart via iommu <iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>]

Hey,

I’ve been investigating the relationship between iommu groups and domains 
on our systems and have a few question. Why does the intel iommu code not 
allow allocating IOMMU_DOMAIN_DMA? Returning NULL when given this domain 
type has the side effect that the default_domain for an iommu group is not 
set, which, when using for e.g. dma_map_ops.map_page, means a domain is 
allocated per device.

This seems to be the opposite behaviour to the AMD iommu code which 
supports allocating an IOMMU_DOMAIN_DMA and will only look to the iommu 
group if a domain is not attached to the device rather than allocating a 
new one. On AMD every device in an iommu group will share the same domain.

Appended is what I think a patch to implement domain_alloc for 
IOMMU_DOMAIN_DMA and also IOMMU_DOMAIN_IDENTITY would look like. Testing 
shows each device in a group will share a domain by default, it also 
allows allocating a new dma domain that can be successfully attached to a 
group with iommu_attach_group.

Looking for comment on why the behaviour is how it is currently and if 
there are any issues with the solution I’ve been testing.

Cheers,
James.


diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index bff2abd6..3a58389f 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -5170,10 +5170,15 @@ static struct iommu_domain *intel_iommu_domain_alloc(unsigned type)
 	struct dmar_domain *dmar_domain;
 	struct iommu_domain *domain;
 
-	if (type != IOMMU_DOMAIN_UNMANAGED)
+	if (type == IOMMU_DOMAIN_UNMANAGED)
+		dmar_domain = alloc_domain(DOMAIN_FLAG_VIRTUAL_MACHINE);
+	else if(type == IOMMU_DOMAIN_DMA)
+		dmar_domain = alloc_domain(0);
+	else if(type == IOMMU_DOMAIN_IDENTITY)
+		dmar_domain = alloc_domain(DOMAIN_FLAG_STATIC_IDENTITY);
+	else
 		return NULL;
 
-	dmar_domain = alloc_domain(DOMAIN_FLAG_VIRTUAL_MACHINE);
 	if (!dmar_domain) {
 		pr_err("Can't allocate dmar_domain\n");
 		return NULL;
@@ -5186,9 +5191,12 @@ static struct iommu_domain *intel_iommu_domain_alloc(unsigned type)
 	domain_update_iommu_cap(dmar_domain);
 
 	domain = &dmar_domain->domain;
-	domain->geometry.aperture_start = 0;
-	domain->geometry.aperture_end   = __DOMAIN_MAX_ADDR(dmar_domain->gaw);
-	domain->geometry.force_aperture = true;
+
+	if (type == IOMMU_DOMAIN_UNMANAGED) {
+		domain->geometry.aperture_start = 0;
+		domain->geometry.aperture_end   = __DOMAIN_MAX_ADDR(dmar_domain->gaw);
+		domain->geometry.force_aperture = true;
+	}
 
 	return domain;
 }
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu