fs/resctrl: Add user interface to enable/disable io_alloc feature

AMD's SDCIAE forces all SDCI lines to be placed into the L3 cache portions
identified by the highest-supported L3_MASK_n register, where n is the maximum
supported CLOSID.

To support this, when io_alloc resctrl feature is enabled, reserve the highest
CLOSID exclusively for I/O allocation traffic making it no longer available for
general CPU cache allocation.

Introduce user interface to enable/disable io_alloc feature and encourage users
to enable io_alloc only when running workloads that can benefit from this
functionality. On enable, initialize the io_alloc CLOSID with all usable CBMs
across all the domains.

Since CLOSIDs are managed by resctrl fs, it is least invasive to make "io_alloc
is supported by maximum supported CLOSID" part of the initial resctrl fs
support for io_alloc. Take care to minimally (only in error messages) expose
this use of CLOSID for io_alloc to user space so that this is not required from
other architectures that may support io_alloc differently in the future.

When resctrl is mounted with "-o cdp" to enable code/data prioritization,
there are two L3 resources that can support I/O allocation: L3CODE and
L3DATA.  From resctrl fs perspective the two resources share a CLOSID and
the architecture's available CLOSID are halved to support this.

The architecture's underlying CLOSID used by SDCIAE when CDP is enabled is the
CLOSID associated with the CDP_CODE resource, but from resctrl's perspective
there is only one CLOSID for both CDP_CODE and CDP_DATA. CDP_DATA is thus not
usable for general (CPU) cache allocation nor I/O allocation.

Keep the CDP_CODE and CDP_DATA I/O alloc status in sync to avoid any confusion
to user space.  That is, enabling io_alloc on CDP_CODE does so on CDP_DATA and
vice-versa, and keep the I/O allocation CBMs of CDP_CODE and CDP_DATA in sync.

Signed-off-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://patch.msgid.link/c7d3037795e653e22b02d8fc73ca80d9b075031c.1762995456.git.babu.moger@amd.com

1 files changed, 30 insertions, 0 deletions

diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index 108995640ca5..91c71e254bbd 100644
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -73,6 +73,11 @@ The 'info' directory contains information about the enabled
 resources. Each resource has its own subdirectory. The subdirectory
 names reflect the resource names.
 
+Most of the files in the resource's subdirectory are read-only, and
+describe properties of the resource. Resources that support global
+configuration options also include writable files that can be used
+to modify those settings.
+
 Each subdirectory contains the following files with respect to
 allocation:
 
@@ -152,6 +157,31 @@ related to allocation:
 			"not supported":
 			      Support not available for this resource.
 
+		The feature can be modified by writing to the interface, for example:
+
+		To enable::
+
+			# echo 1 > /sys/fs/resctrl/info/L3/io_alloc
+
+		To disable::
+
+			# echo 0 > /sys/fs/resctrl/info/L3/io_alloc
+
+		The underlying implementation may reduce resources available to
+		general (CPU) cache allocation. See architecture specific notes
+		below. Depending on usage requirements the feature can be enabled
+		or disabled.
+
+		On AMD systems, io_alloc feature is supported by the L3 Smart
+		Data Cache Injection Allocation Enforcement (SDCIAE). The CLOSID for
+		io_alloc is the highest CLOSID supported by the resource. When
+		io_alloc is enabled, the highest CLOSID is dedicated to io_alloc and
+		no longer available for general (CPU) cache allocation. When CDP is
+		enabled, io_alloc routes I/O traffic using the highest CLOSID allocated
+		for the instruction cache (CDP_CODE), making this CLOSID no longer
+		available for general (CPU) cache allocation for both the CDP_CODE
+		and CDP_DATA resources.
+
 Memory bandwidth(MB) subdirectory contains the following files
 with respect to allocation: