[PATCH v2 4/8] ACPI/IORT: Support CANWBS memory access flag

Jason Gunthorpe
2024-08-27T15:56:55Z

From: Nicolin Chen <nicolinc@nvidia.com>

The IORT spec, Issue E.f (April 2024), adds a new CANWBS bit to the Memory
Access Flag field in the Memory Access Properties table, mainly for a PCI
Root Complex.

This CANWBS defines the coherency of memory accesses to be not marked IOWB
cacheable/shareable. Its value further implies the coherency impact from a
pair of mismatched memory attributes (e.g. in a nested translation case):
0x0: Use of mismatched memory attributes for accesses made by this
device may lead to a loss of coherency.
0x1: Coherency of accesses made by this device to locations in
Conventional memory are ensured as follows, even if the memory
attributes for the accesses presented by the device or provided by
the SMMU are different from Inner and Outer Write-back cacheable,
Shareable.

Note that the loss of coherency on a CANWBS-unsupported HW typically could
occur to an SMMU that doesn't implement the S2FWB feature where additional
cache flush operations would be required to prevent that from happening.

Add a new ACPI_IORT_MF_CANWBS flag and set IOMMU_FWSPEC_PCI_RC_CANWBS upon
the presence of this new flag.

CANWBS and S2FWB are similar features, in that they both guarantee the VM
can not violate coherency, however S2FWB can be bypassed by PCI No Snoop
TLPs, while CANWBS cannot. Thus CANWBS meets the requirements to set
IOMMU_CAP_ENFORCE_CACHE_COHERENCY.

Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
drivers/acpi/arm64/iort.c | 13 +++++++++++++
include/linux/iommu.h | 2 ++
2 files changed, 15 insertions(+)

diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
index 1b39e9ae7ac178..52f5836fa888db 100644
--- a/drivers/acpi/arm64/iort.c
+++ b/drivers/acpi/arm64/iort.c
@@ -1218,6 +1218,17 @@ static bool iort_pci_rc_supports_ats(struct acpi_iort_node *node)
return pci_rc->ats_attribute & ACPI_IORT_ATS_SUPPORTED;
}

+static bool iort_pci_rc_supports_canwbs(struct acpi_iort_node *node)
+{
+ struct acpi_iort_memory_access *memory_access;
+ struct acpi_iort_root_complex *pci_rc;
+
+ pci_rc = (struct acpi_iort_root_complex *)node->node_data;
+ memory_access =
+ (struct acpi_iort_memory_access *)&pci_rc->memory_properties;
+ return memory_access->memory_flags & ACPI_IORT_MF_CANWBS;
+}
+
static int iort_iommu_xlate(struct device *dev, struct acpi_iort_node *node,
u32 streamid)
{
@@ -1335,6 +1346,8 @@ int iort_iommu_configure_id(struct device *dev, const u32 *id_in)
fwspec = dev_iommu_fwspec_get(dev);
if (fwspec && iort_pci_rc_supports_ats(node))
fwspec->flags |= IOMMU_FWSPEC_PCI_RC_ATS;
+ if (fwspec && iort_pci_rc_supports_canwbs(node))
+ fwspec->flags |= IOMMU_FWSPEC_PCI_RC_CANWBS;
} else {
node = iort_scan_node(ACPI_IORT_NODE_NAMED_COMPONENT,
iort_match_node_callback, dev);
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 15d7657509f662..d1660ec23f263b 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -993,6 +993,8 @@ struct iommu_fwspec {

/* ATS is supported */
#define IOMMU_FWSPEC_PCI_RC_ATS (1 << 0)
+/* CANWBS is supported */
+#define IOMMU_FWSPEC_PCI_RC_CANWBS (1 << 1)

/*
* An iommu attach handle represents a relationship between an iommu domain
--
2.46.0