PCI: Extend ACS configurability

PCIe ACS settings control the level of isolation and the possible P2P paths
between devices. With greater isolation the kernel will create smaller
iommu_groups and with less isolation there is more HW that can achieve P2P
transfers. From a virtualization perspective all devices in the same
iommu_group must be assigned to the same VM as they lack security
isolation.

There is no way for the kernel to automatically know the correct ACS
settings for any given system and workload. Existing command line options
(e.g., disable_acs_redir) allow only for large scale change, disabling all
isolation, but this is not sufficient for more complex cases.

Add a kernel command-line option 'config_acs' to directly control all the
ACS bits for specific devices, which allows the operator to setup the right
level of isolation to achieve the desired P2P configuration.  The
definition is future proof; when new ACS bits are added to the spec the
open syntax can be extended.

ACS needs to be setup early in the kernel boot as the ACS settings affect
how iommu_groups are formed. iommu_group formation is a one time event
during initial device discovery, so changing ACS bits after kernel boot can
result in an inaccurate view of the iommu_groups compared to the current
isolation configuration.

ACS applies to PCIe Downstream Ports and multi-function devices.  The
default ACS settings are strict and deny any direct traffic between two
functions. This results in the smallest iommu_group the HW can support.
Frequently these values result in slow or non-working P2PDMA.

ACS offers a range of security choices controlling how traffic is
allowed to go directly between two devices. Some popular choices:

  - Full prevention

  - Translated requests can be direct, with various options

  - Asymmetric direct traffic, A can reach B but not the reverse

  - All traffic can be direct

Along with some other less common ones for special topologies.

The intention is that this option would be used with expert knowledge of
the HW capability and workload to achieve the desired configuration.

Link: https://lore.kernel.org/r/20240625153150.159310-1-vidyas@nvidia.com
Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
[bhelgaas: add example, tidy printk formats]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 500cfa776225..b2057241ea6c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4619,6 +4619,38 @@
 				bridges without forcing it upstream. Note:
 				this removes isolation between devices and
 				may put more devices in an IOMMU group.
+		config_acs=
+				Format:
+				<ACS flags>@<pci_dev>[; ...]
+				Specify one or more PCI devices (in the format
+				specified above) optionally prepended with flags
+				and separated by semicolons. The respective
+				capabilities will be enabled, disabled or
+				unchanged based on what is specified in
+				flags.
+
+				ACS Flags is defined as follows:
+				  bit-0 : ACS Source Validation
+				  bit-1 : ACS Translation Blocking
+				  bit-2 : ACS P2P Request Redirect
+				  bit-3 : ACS P2P Completion Redirect
+				  bit-4 : ACS Upstream Forwarding
+				  bit-5 : ACS P2P Egress Control
+				  bit-6 : ACS Direct Translated P2P
+				Each bit can be marked as:
+				  '0' – force disabled
+				  '1' – force enabled
+				  'x' – unchanged
+				For example,
+				  pci=config_acs=10x
+				would configure all devices that support
+				ACS to enable P2P Request Redirect, disable
+				Translation Blocking, and leave Source
+				Validation unchanged from whatever power-up
+				or firmware set it to.
+
+				Note: this may remove isolation between devices
+				and may put more devices in an IOMMU group.
 		force_floating	[S390] Force usage of floating interrupts.
 		nomio		[S390] Do not use MIO instructions.
 		norid		[S390] ignore the RID field and force use of