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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 18:49:45 +0000
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+===================================
+DSCR (Data Stream Control Register)
+===================================
+
+DSCR register in powerpc allows user to have some control of prefetch of data
+stream in the processor. Please refer to the ISA documents or related manual
+for more detailed information regarding how to use this DSCR to attain this
+control of the prefetches . This document here provides an overview of kernel
+support for DSCR, related kernel objects, it's functionalities and exported
+user interface.
+
+(A) Data Structures:
+
+ (1) thread_struct::
+
+ dscr /* Thread DSCR value */
+ dscr_inherit /* Thread has changed default DSCR */
+
+ (2) PACA::
+
+ dscr_default /* per-CPU DSCR default value */
+
+ (3) sysfs.c::
+
+ dscr_default /* System DSCR default value */
+
+(B) Scheduler Changes:
+
+ Scheduler will write the per-CPU DSCR default which is stored in the
+ CPU's PACA value into the register if the thread has dscr_inherit value
+ cleared which means that it has not changed the default DSCR till now.
+ If the dscr_inherit value is set which means that it has changed the
+ default DSCR value, scheduler will write the changed value which will
+ now be contained in thread struct's dscr into the register instead of
+ the per-CPU default PACA based DSCR value.
+
+ NOTE: Please note here that the system wide global DSCR value never
+ gets used directly in the scheduler process context switch at all.
+
+(C) SYSFS Interface:
+
+ - Global DSCR default: /sys/devices/system/cpu/dscr_default
+ - CPU specific DSCR default: /sys/devices/system/cpu/cpuN/dscr
+
+ Changing the global DSCR default in the sysfs will change all the CPU
+ specific DSCR defaults immediately in their PACA structures. Again if
+ the current process has the dscr_inherit clear, it also writes the new
+ value into every CPU's DSCR register right away and updates the current
+ thread's DSCR value as well.
+
+ Changing the CPU specific DSCR default value in the sysfs does exactly
+ the same thing as above but unlike the global one above, it just changes
+ stuff for that particular CPU instead for all the CPUs on the system.
+
+(D) User Space Instructions:
+
+ The DSCR register can be accessed in the user space using any of these
+ two SPR numbers available for that purpose.
+
+ (1) Problem state SPR: 0x03 (Un-privileged, POWER8 only)
+ (2) Privileged state SPR: 0x11 (Privileged)
+
+ Accessing DSCR through privileged SPR number (0x11) from user space
+ works, as it is emulated following an illegal instruction exception
+ inside the kernel. Both mfspr and mtspr instructions are emulated.
+
+ Accessing DSCR through user level SPR (0x03) from user space will first
+ create a facility unavailable exception. Inside this exception handler
+ all mfspr instruction based read attempts will get emulated and returned
+ where as the first mtspr instruction based write attempts will enable
+ the DSCR facility for the next time around (both for read and write) by
+ setting DSCR facility in the FSCR register.
+
+(E) Specifics about 'dscr_inherit':
+
+ The thread struct element 'dscr_inherit' represents whether the thread
+ in question has attempted and changed the DSCR itself using any of the
+ following methods. This element signifies whether the thread wants to
+ use the CPU default DSCR value or its own changed DSCR value in the
+ kernel.
+
+ (1) mtspr instruction (SPR number 0x03)
+ (2) mtspr instruction (SPR number 0x11)
+ (3) ptrace interface (Explicitly set user DSCR value)
+
+ Any child of the process created after this event in the process inherits
+ this same behaviour as well.