[PATCH v6 20/40] arm_mpam: resctrl: Add CDP emulation

[Gavin Shan]

On 3/14/26 12:45 AM, Ben Horgan wrote:
> From: James Morse <james.morse at arm.com>
> 
> Intel RDT's CDP feature allows the cache to use a different control value
> depending on whether the accesses was for instruction fetch or a data
> access. MPAM's equivalent feature is the other way up: the CPU assigns a
> different partid label to traffic depending on whether it was instruction
> fetch or a data access, which causes the cache to use a different control
> value based solely on the partid.
> 
> MPAM can emulate CDP, with the side effect that the alternative partid is
> seen by all MSC, it can't be enabled per-MSC.
> 
> Add the resctrl hooks to turn this on or off. Add the helpers that match a
> closid against a task, which need to be aware that the value written to
> hardware is not the same as the one resctrl is using.
> 
> Update the 'arm64_mpam_global_default' variable the arch code uses during
> context switch to know when the per-cpu value should be used instead. Also,
> update these per-cpu values and sync the resulting mpam partid/pmg
> configuration to hardware.
> 
> resctrl can enable CDP for L2 caches, L3 caches or both. When it is enabled
> by one and not the other MPAM globally enabled CDP but hides the effect
> on the other cache resource. This hiding is possible as CPOR is the only
> supported cache control and that uses a resource bitmap; two partids with
> the same bitmap act as one.
> 
> Awkwardly, the MB controls don't implement CDP and CDP can't be hidden as
> the memory bandwidth control is a maximum per partid which can't be
> modelled with more partids. If the total maximum is used for both the data
> and instruction partids then then the maximum may be exceeded and if it is
> split in two then the one using more bandwidth will hit a lower
> limit. Hence, hide the MB controls completely if CDP is enabled for any
> resource.
> 
> Tested-by: Gavin Shan <gshan at redhat.com>
> Tested-by: Shaopeng Tan <tan.shaopeng at jp.fujitsu.com>
> Tested-by: Peter Newman <peternewman at google.com>
> Tested-by: Zeng Heng <zengheng4 at huawei.com>
> Tested-by: Punit Agrawal <punit.agrawal at oss.qualcomm.com>
> Cc: Dave Martin <Dave.Martin at arm.com>
> Cc: Amit Singh Tomar <amitsinght at marvell.com>
> Reviewed-by: Zeng Heng <zengheng4 at huawei.com>
> Reviewed-by: Shaopeng Tan <tan.shaopeng at jp.fujitsu.com>
> Reviewed-by: Jonathan Cameron <jonathan.cameron at huawei.com>
> Signed-off-by: James Morse <james.morse at arm.com>
> Signed-off-by: Ben Horgan <ben.horgan at arm.com>
> ---
> Changes since rfc:
> Fail cdp initialisation if there is only one partid
> Correct data/code confusion
> 
> Changes since v2:
> Don't include unused header
> 
> Changes since v3:
> Update the per-cpu values and sync to h/w
> 
> Changes since v4:
> Enable separately for L2 and L3
> Disable MB controls if CDP enabled
> Consider cdp hiding in resctrl_arch_update_one()
> 
> Changes since v5:
> Update comment on call sites
> ---
>   arch/arm64/include/asm/mpam.h   |   1 +
>   drivers/resctrl/mpam_internal.h |   1 +
>   drivers/resctrl/mpam_resctrl.c  | 122 ++++++++++++++++++++++++++++++++
>   include/linux/arm_mpam.h        |   2 +
>   4 files changed, 126 insertions(+)
> 
Reviewed-by: Gavin Shan <gshan at redhat.com>