[PATCH V2] powerpc: Support for generic TIF bits

["Mukesh Kumar Chaurasiya (IBM)" <mkchauras@gmail.com>]

powerpc already uses GENERIC_ENTRY for the irq entry/exit path.  That
means the generic entry code handles all the arch-independent
thread_info flags (TIF_SIGPENDING, TIF_NOTIFY_RESUME, TIF_NOTIFY_SIGNAL,
TIF_NEED_RESCHED, TIF_NEED_RESCHED_LAZY, TIF_UPROBE, TIF_PATCH_PENDING,
TIF_MEMDIE, TIF_RSEQ, TIF_HRTIMER_REARM, TIF_USER_RETURN_NOTIFY) on
behalf of the arch.  The only remaining blocker was that powerpc defined
its own bit numbers for those flags, which conflicted with the layout
required by the common header include/asm-generic/thread_info_tif.h.

This patch selects HAVE_GENERIC_TIF_BITS and switches powerpc's
thread_info.h over to include <asm-generic/thread_info_tif.h> for the
arch-independent bits, using HAVE_TIF_NEED_RESCHED_LAZY to opt in to
the lazy-preemption flag.  The powerpc-private bits (TIF_RESTORE_TM,
TIF_SINGLESTEP, TIF_RESTOREALL, TIF_NOERROR, TIF_EMULATE_STACK_STORE,
TIF_ELF2ABI, TIF_POLLING_NRFLAG, TIF_32BIT, TIF_SYSCALL_RET) are
renumbered to start at bit 16, which is the reserved range for arch use.

Enabling HAVE_GENERIC_TIF_BITS unlocks the following features that were
previously unavailable on powerpc because their Kconfig dependencies
could not be satisfied:

  1. RSEQ_SLICE_EXTENSION (CONFIG_RSEQ_SLICE_EXTENSION)
     Allows userspace to register an RSEQ critical section and request
     a limited time-slice extension when the task is about to be
     preempted.  The kernel grants the extension (via the RSEQ shared
     data ABI) so the critical section can complete, avoiding the
     thundering-herd problem where many threads spin on the same
     lock/resource after a preemption.  This feature requires a
     dedicated TIF_RSEQ bit, which only exists when HAVE_GENERIC_TIF_BITS
     is set.  Depends on: RSEQ && HIGH_RES_TIMERS && GENERIC_ENTRY &&
     HAVE_GENERIC_TIF_BITS.

  2. HRTIMER_REARM_DEFERRED (CONFIG_HRTIMER_REARM_DEFERRED)
     Defers re-arming of the hrtimer interrupt to the return-to-user
     path, using the dedicated TIF_HRTIMER_REARM bit.  This avoids
     redundant hrtimer_interrupt() invocations within the same tick
     period and reduces latency jitter for high-resolution timer users
     (e.g., PREEMPT_RT workloads, audio, real-time tasks).  Depends on:
     GENERIC_ENTRY && HAVE_GENERIC_TIF_BITS && HIGH_RES_TIMERS &&
     SCHED_HRTICK.

  3. Dedicated TIF_RSEQ flag (fast-path RSEQ handling)
     Without HAVE_GENERIC_TIF_BITS, TIF_RSEQ falls back to aliasing
     TIF_NOTIFY_RESUME, forcing the generic entry slow-path to run on
     every return-to-user for all RSEQ-registered threads.  With the
     dedicated bit, the fast path can skip RSEQ work unless the flag
     is actually set, reducing overhead in the common case.

  4. TIF_USER_RETURN_NOTIFY
     The generic bit for the user-return-notifier subsystem
     (kernel/user-return-notifier.c).  Used by KVM on x86 to lazily
     restore guest MSRs; having the generic bit available means future
     powerpc KVM work can use the same infrastructure.

The arch-specific bits that do not exist in the generic header retain
their private numbering in the powerpc-reserved range (bits 16+), so
no existing powerpc functionality is affected.

Tested on powerpc (POWER11 pSeries CONFIG_RSEQ=y +
CONFIG_RSEQ_SLICE_EXTENSION=y + CONFIG_HRTIMER_REARM_DEFERRED=y):

  * tools/testing/selftests/rseq/ -- all tests pass (basic_test,
    param_test, basic_percpu_ops_test, syscall_errors_test,
    slice_test via run_timeslice_test.sh)
  * tools/testing/selftests/timers/hrtimer-rearm.sh -- deferred=1
    events observed; no new dmesg warnings
  * Boot and basic workload testing shows no regression

Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
---
This patch is applicabe over
https://lore.kernel.org/all/961c058f3f3e03045552186c083a68a0fde3f43f.1791975972.git.msuchanek@suse.de/

Changelog:

V1 -> V2:
 - Commit message updated
V1: https://lore.kernel.org/all/20261021144453.232372-2-mkchauras@gmail.com/

 arch/powerpc/Kconfig                   |  1 +
 arch/powerpc/include/asm/thread_info.h | 36 +++++++++-----------------
 2 files changed, 13 insertions(+), 24 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 7f7910f38a67..e5527b56159a 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -255,6 +255,7 @@ config PPC
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER		if !COMPILE_TEST && (PPC64 || (PPC32 && CC_IS_GCC))
 	select HAVE_GCC_PLUGINS
+	select HAVE_GENERIC_TIF_BITS
 	select HAVE_HARDLOCKUP_DETECTOR_ARCH	if PPC_BOOK3S_64 && SMP
 	select HAVE_HARDLOCKUP_DETECTOR_PERF	if PERF_EVENTS && HAVE_PERF_EVENTS_NMI
 	select HAVE_HW_BREAKPOINT		if PERF_EVENTS && (PPC_BOOK3S || PPC_8xx)
diff --git a/arch/powerpc/include/asm/thread_info.h b/arch/powerpc/include/asm/thread_info.h
index 4b7ec1db3034..4df3f4b5bd56 100644
--- a/arch/powerpc/include/asm/thread_info.h
+++ b/arch/powerpc/include/asm/thread_info.h
@@ -93,44 +93,32 @@ void arch_setup_new_exec(void);
 
 #endif /* __ASSEMBLER__ */
 
+#define HAVE_TIF_NEED_RESCHED_LAZY
+
+#include <asm-generic/thread_info_tif.h>
 /*
  * thread information flag bit numbers
  */
-#define TIF_SIGPENDING		1	/* signal pending */
-#define TIF_NEED_RESCHED	2	/* rescheduling necessary */
-#define TIF_NOTIFY_SIGNAL	3	/* signal notifications exist */
-#define TIF_RESTORE_TM		5	/* need to restore TM FP/VEC/VSX */
-#define TIF_PATCH_PENDING	6	/* pending live patching update */
-#define TIF_SINGLESTEP		8	/* singlestepping active */
-#define TIF_NEED_RESCHED_LAZY	9       /* Scheduler driven lazy preemption */
-#define TIF_RESTOREALL		11	/* Restore all regs (implies NOERROR) */
-#define TIF_NOERROR		12	/* Force successful syscall return */
-#define TIF_NOTIFY_RESUME	13	/* callback before returning to user */
-#define TIF_UPROBE		14	/* breakpointed or single-stepping */
-#define TIF_EMULATE_STACK_STORE	16	/* Is an instruction emulation
+#define TIF_RESTORE_TM		16	/* need to restore TM FP/VEC/VSX */
+#define TIF_SINGLESTEP		17	/* singlestepping active */
+#define TIF_RESTOREALL		18	/* Restore all regs (implies NOERROR) */
+#define TIF_NOERROR		19	/* Force successful syscall return */
+#define TIF_EMULATE_STACK_STORE	20	/* Is an instruction emulation
 						for stack store? */
-#define TIF_MEMDIE		17	/* is terminating due to OOM killer */
 #if defined(CONFIG_PPC64)
-#define TIF_ELF2ABI		18	/* function descriptors must die! */
+#define TIF_ELF2ABI		21	/* function descriptors must die! */
 #endif
-#define TIF_POLLING_NRFLAG	19	/* true if poll_idle() is polling TIF_NEED_RESCHED */
-#define TIF_32BIT		20	/* 32 bit binary */
-#define TIF_SYSCALL_RET		21	/* syscall error value set */
+#define TIF_POLLING_NRFLAG	22	/* true if poll_idle() is polling TIF_NEED_RESCHED */
+#define TIF_32BIT		23	/* 32 bit binary */
+#define TIF_SYSCALL_RET		24	/* syscall error value set */
 
 /* as above, but as bit values */
-#define _TIF_SIGPENDING		(1<<TIF_SIGPENDING)
-#define _TIF_NEED_RESCHED	(1<<TIF_NEED_RESCHED)
-#define _TIF_NEED_RESCHED_LAZY	(1<<TIF_NEED_RESCHED_LAZY)
-#define _TIF_NOTIFY_SIGNAL	(1<<TIF_NOTIFY_SIGNAL)
 #define _TIF_POLLING_NRFLAG	(1<<TIF_POLLING_NRFLAG)
 #define _TIF_32BIT		(1<<TIF_32BIT)
 #define _TIF_RESTORE_TM		(1<<TIF_RESTORE_TM)
-#define _TIF_PATCH_PENDING	(1<<TIF_PATCH_PENDING)
 #define _TIF_SINGLESTEP		(1<<TIF_SINGLESTEP)
 #define _TIF_RESTOREALL		(1<<TIF_RESTOREALL)
 #define _TIF_NOERROR		(1<<TIF_NOERROR)
-#define _TIF_NOTIFY_RESUME	(1<<TIF_NOTIFY_RESUME)
-#define _TIF_UPROBE		(1<<TIF_UPROBE)
 #define _TIF_EMULATE_STACK_STORE	(1<<TIF_EMULATE_STACK_STORE)
 
 #define _TIF_USER_WORK_MASK	(_TIF_SIGPENDING | _TIF_NEED_RESCHED | \
-- 
2.56.0