Re: [PATCH] pci: Enable overrides for missing ACS capabilities

From: Don Dutile
Date: Mon Jun 17 2013 - 11:01:14 EST

On 05/30/2013 02:40 PM, Alex Williamson wrote:
PCIe ACS (Access Control Services) is the PCIe 2.0+ feature that
allows us to control whether transactions are allowed to be redirected
in various subnodes of a PCIe topology. For instance, if two
endpoints are below a root port or downsteam switch port, the
downstream port may optionally redirect transactions between the
devices, bypassing upstream devices. The same can happen internally
on multifunction devices. The transaction may never be visible to the
upstream devices.

One upstream device that we particularly care about is the IOMMU. If
a redirection occurs in the topology below the IOMMU, then the IOMMU
cannot provide isolation between devices. This is why the PCIe spec
encourages topologies to include ACS support. Without it, we have to
assume peer-to-peer DMA within a hierarchy can bypass IOMMU isolation.

Unfortunately, far too many topologies do not support ACS to make this
a steadfast requirement. Even the latest chipsets from Intel are only
sporadically supporting ACS. We have trouble getting interconnect
vendors to include the PCIe spec required PCIe capability, let alone
suggested features.

Therefore, we need to add some flexibility. The pcie_acs_override=
boot option lets users opt-in specific devices or sets of devices to
assume ACS support. The "downstream" option assumes full ACS support
on root ports and downstream switch ports. The "multifunction"
option assumes the subset of ACS features available on multifunction
endpoints and upstream switch ports are supported. The "id:nnnn:nnnn"
option enables ACS support on devices matching the provided vendor
and device IDs, allowing more strategic ACS overrides. These options
may be combined in any order. A maximum of 16 id specific overrides
are available. It's suggested to use the most limited set of options
necessary to avoid completely disabling ACS across the topology.

Note to hardware vendors, we have facilities to permanently quirk
specific devices which enforce isolation but not provide an ACS
capability. Please contact me to have your devices added and save
your customers the hassle of this boot option.

Signed-off-by: Alex Williamson<alex.williamson@xxxxxxxxxx>
Documentation/kernel-parameters.txt | 10 +++
drivers/pci/quirks.c | 102 +++++++++++++++++++++++++++++++++++
2 files changed, 112 insertions(+)

Feel free to add my ack.

I like the fact that all of this code is in quirks, and not sprinkled btwn pci core & quirks.
As we have discovered, even common, ACS-compatilbe x86 chipsets are out there w/o ACS caps.
Additionally, an unclear area of the spec has some vendors providing 'null ACS' caps,
which is suppose to be interpreted as no-peer-to-peer DMA; this latter 'feature' is being
brought up to PCI-SIG to get a note added to SRIOV spec in the ACS-related material to clarify.

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 47bb23c..a60e6ad 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2349,6 +2349,16 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
nomsi Do not use MSI for native PCIe PME signaling (this makes
all PCIe root ports use INTx for all services).

+ pcie_acs_override =
+ [PCIE] Override missing PCIe ACS support for:
+ downstream
+ All downstream ports - full ACS capabilties
+ multifunction
+ All multifunction devices - multifunction ACS subset
+ id:nnnn:nnnn
+ Specfic device - full ACS capabilities
+ Specified as vid:did (vendor/device ID) in hex
pcmv= [HW,PCMCIA] BadgePAD 4

pd. [PARIDE]
diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
index 0369fb6..c7609f6 100644
--- a/drivers/pci/quirks.c
+++ b/drivers/pci/quirks.c
@@ -3292,11 +3292,113 @@ struct pci_dev *pci_get_dma_source(struct pci_dev *dev)
return pci_dev_get(dev);

+static bool acs_on_downstream;
+static bool acs_on_multifunction;
+#define NUM_ACS_IDS 16
+struct acs_on_id {
+ unsigned short vendor;
+ unsigned short device;
At first, I wasn't sure if vid/did would be sufficient, but convinced myself
that a sub-vid/did that differed amongst sub-vid's, even if they added/modified
ACS cap, the ACS-cap-existence test in pcie_acs_overrides() will not exercise
any override for that variant, and both bases -- newer/sub-vid modified devices
w/ACS, and vid/did w/o ACS will be handled properly.
So, another reason for the ack.

+static struct acs_on_id acs_on_ids[NUM_ACS_IDS];
+static u8 max_acs_id;
+static __init int pcie_acs_override_setup(char *p)
+ if (!p)
+ return -EINVAL;
+ while (*p) {
+ if (!strncmp(p, "downstream", 10))
+ acs_on_downstream = true;
+ if (!strncmp(p, "multifunction", 13))
+ acs_on_multifunction = true;
+ if (!strncmp(p, "id:", 3)) {
+ char opt[5];
+ int ret;
+ long val;
+ if (max_acs_id>= NUM_ACS_IDS - 1) {
+ pr_warn("Out of PCIe ACS override slots (%d)\n",
+ goto next;
+ }
+ p += 3;
+ snprintf(opt, 5, "%s", p);
+ ret = kstrtol(opt, 16,&val);
+ if (ret) {
+ pr_warn("PCIe ACS ID parse error %d\n", ret);
+ goto next;
+ }
+ acs_on_ids[max_acs_id].vendor = val;
+ p += strcspn(p, ":");
+ if (*p != ':') {
+ pr_warn("PCIe ACS invalid ID\n");
+ goto next;
+ }
+ p++;
+ snprintf(opt, 5, "%s", p);
+ ret = kstrtol(opt, 16,&val);
+ if (ret) {
+ pr_warn("PCIe ACS ID parse error %d\n", ret);
+ goto next;
+ }
+ acs_on_ids[max_acs_id].device = val;
+ max_acs_id++;
+ }
+ p += strcspn(p, ",");
+ if (*p == ',')
+ p++;
+ }
+ if (acs_on_downstream || acs_on_multifunction || max_acs_id)
+ pr_warn("Warning: PCIe ACS overrides enabled; This may allow non-IOMMU protected peer-to-peer DMA\n");
+ return 0;
+early_param("pcie_acs_override", pcie_acs_override_setup);
+static int pcie_acs_overrides(struct pci_dev *dev, u16 acs_flags)
+ int i;
+ /* Never override ACS for legacy devices or devices with ACS caps */
+ if (!pci_is_pcie(dev) ||
+ pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ACS))
+ return -ENOTTY;
+ for (i = 0; i< max_acs_id; i++)
+ if (acs_on_ids[i].vendor == dev->vendor&&
+ acs_on_ids[i].device == dev->device)
+ return 1;
+ switch (pci_pcie_type(dev)) {
+ if (acs_on_downstream)
+ return 1;
+ break;
+ if (acs_on_multifunction&& dev->multifunction)
+ return 1;
+ }
+ return -ENOTTY;
static const struct pci_dev_acs_enabled {
u16 vendor;
u16 device;
int (*acs_enabled)(struct pci_dev *dev, u16 acs_flags);
} pci_dev_acs_enabled[] = {
+ { PCI_ANY_ID, PCI_ANY_ID, pcie_acs_overrides },
{ 0 }

Would like to see this list filled out asap to avoid cmdline workarounds! ;-)
wait! -- did I say that? I want quirks?!? ... ;-)
Honestly, I'd rather see the vid/did here then numerous kernel cmdline args being added.

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