Hi Reinette,


On 7/21/25 18:28, Reinette Chatre wrote:

Hi Babu,

On 7/10/25 10:16 AM, Babu Moger wrote:

quoted

This series adds the support for L3 Smart Data Cache Injection Allocation
Enforcement (SDCIAE) to resctrl infrastructure. It is referred to as
"io_alloc" in resctrl subsystem.

Upcoming AMD hardware implements Smart Data Cache Injection (SDCI).
Smart Data Cache Injection (SDCI) is a mechanism that enables direct
insertion of data from I/O devices into the L3 cache. By directly caching
data from I/O devices rather than first storing the I/O data in DRAM, SDCI
reduces demands on DRAM bandwidth and reduces latency to the processor
consuming the I/O data.

The SDCIAE (SDCI Allocation Enforcement) PQE feature allows system software
to control the portion of the L3 cache used for SDCI devices.

When enabled, SDCIAE forces all SDCI lines to be placed into the L3 cache
partitions identified by the highest-supported L3_MASK_n register, where n
is the maximum supported CLOSID.

Even though this CLOSID use is unique to AMD this implementation makes it part
of resctrl fs's support of io_alloc. It is confusing to have architectural
specific features be handled by resctrl fs so I think it will be useful to
add a snippet here to help folks trying to decipher this work. Consider
for example a snippet like:

  Since CLOSIDs are managed by resctrl fs it is least invasive to make
  the "io_alloc is supported by maximum supported CLOSID" part of the
  initial resctrl fs support for io_alloc. Take care not to 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.

Sure.

quoted

The feature details are documented in the APM listed below [1].
[1] AMD64 Architecture Programmer's Manual Volume 2: System Programming
Publication # 24593 Revision 3.41 section 19.4.7 L3 Smart Data Cache
Injection Allocation Enforcement (SDCIAE)
Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537

The feature requires linux support of TPH (TLP Processing Hints).
The support is available in linux kernel after the commit
48d0fd2b903e3 ("PCI/TPH: Add TPH documentation")

The patches are based on top of commit (6.16.0-rc5)
commit b4ec95e3bc3f ("Merge x86/microcode into tip/master")

# Linux Implementation

Feature adds following interface files when the resctrl "io_alloc" feature
is supported on the resource:

/sys/fs/resctrl/info/L3/io_alloc: Report the feature status. Enable/disable the
				  feature by writing to the interface.

/sys/fs/resctrl/info/L3/io_alloc_cbm:  List the Capacity Bit Masks (CBMs) available
				       for I/O devices when io_alloc feature is enabled.
				       Configure the CBM by writing to the interface.

When CDP is enabled, these files will be created both in L3CODE and L3DATA.

"will be" -> "are"

Sure.

quoted

# Examples:

a. Check if io_alloc feature is available
	# mount -t resctrl resctrl /sys/fs/resctrl/

	# cat /sys/fs/resctrl/info/L3/io_alloc
	disabled

b. Enable the io_alloc feature. 

	# echo 1 > /sys/fs/resctrl/info/L3/io_alloc 
	# cat /sys/fs/resctrl/info/L3/io_alloc
	enabled

c. Check the CBM values for the io_alloc feature.

	# cat /sys/fs/resctrl/info/L3/io_alloc_cbm 
	0=ffff;1=ffff

d. Change the CBM value for the domain 1:

"for the domain 1" -> "of domain 1"?

Sure.

(nit: inconsistent use of "." vs. ":" in items)

Fixed it.

quoted

	# echo 1=FF > /sys/fs/resctrl/info/L3/io_alloc_cbm

	# cat /sys/fs/resctrl/info/L3/io_alloc_cbm 
	0=ffff;1=00ff

d. Disable io_alloc feature and exit.

	# echo 0 > /sys/fs/resctrl/info/L3/io_alloc 
	# cat /sys/fs/resctrl/info/L3/io_alloc
	disabled

	# umount /sys/fs/resctrl/

---

Reinette

-- 
Thanks
Babu Moger