linux-kernel - [PATCH v2] sparc/vdso: Add helper function for 64-bit right shift on 32-bit target

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Message-Id: <20240808-sparc-shr64-v2-1-fd18f1b2cea9@protonmail.com>
Date: Thu, 08 Aug 2024 09:05:00 +0700
From: Koakuma via B4 Relay <devnull+koachan.protonmail.com@...nel.org>
To: "David S. Miller" <davem@...emloft.net>, 
 Andreas Larsson <andreas@...sler.com>, Andy Lutomirski <luto@...nel.org>, 
 Thomas Gleixner <tglx@...utronix.de>, 
 Vincenzo Frascino <vincenzo.frascino@....com>, 
 Nathan Chancellor <nathan@...nel.org>, 
 Nick Desaulniers <ndesaulniers@...gle.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Cc: sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, Koakuma <koachan@...tonmail.com>
Subject: [PATCH v2] sparc/vdso: Add helper function for 64-bit right shift
 on 32-bit target

From: Koakuma <koachan@...tonmail.com>

Add helper function for 64-bit right shift on 32-bit target so that
clang does not emit a runtime library call.

Signed-off-by: Koakuma <koachan@...tonmail.com>
---
Hi~

This adds a small function to do 64-bit right shifts for use in vDSO
code, needed so that clang does not emit a call to runtime library.
---
Changes in v2:
- Move __shr64 to sparc code since there are no other users of it.
- Now that __shr64 is not in portable code, redo it in inline asm for simpler implementation & better performance.
- Link to v1: https://lore.kernel.org/r/20240804-sparc-shr64-v1-1-25050968339a@protonmail.com
---
 arch/sparc/vdso/vclock_gettime.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/arch/sparc/vdso/vclock_gettime.c b/arch/sparc/vdso/vclock_gettime.c
index e794edde6755..79607804ea1b 100644
--- a/arch/sparc/vdso/vclock_gettime.c
+++ b/arch/sparc/vdso/vclock_gettime.c
@@ -86,6 +86,11 @@ notrace static long vdso_fallback_gettimeofday(struct __kernel_old_timeval *tv,
 }
 
 #ifdef	CONFIG_SPARC64
+notrace static __always_inline u64 __shr64(u64 val, int amt)
+{
+	return val >> amt;
+}
+
 notrace static __always_inline u64 vread_tick(void)
 {
 	u64	ret;
@@ -102,6 +107,21 @@ notrace static __always_inline u64 vread_tick_stick(void)
 	return ret;
 }
 #else
+notrace static __always_inline u64 __shr64(u64 val, int amt)
+{
+	u64 ret;
+
+	__asm__ __volatile__("sllx %H1, 32, %%g1\n\t"
+			     "srl %L1, 0, %L1\n\t"
+			     "or %%g1, %L1, %%g1\n\t"
+			     "srlx %%g1, %2, %L0\n\t"
+			     "srlx %L0, 32, %H0"
+			     : "=r" (ret)
+			     : "r" (val), "r" (amt)
+			     : "g1");
+	return ret;
+}
+
 notrace static __always_inline u64 vread_tick(void)
 {
 	register unsigned long long ret asm("o4");
@@ -154,7 +174,7 @@ notrace static __always_inline int do_realtime(struct vvar_data *vvar,
 		ts->tv_sec = vvar->wall_time_sec;
 		ns = vvar->wall_time_snsec;
 		ns += vgetsns(vvar);
-		ns >>= vvar->clock.shift;
+		ns = __shr64(ns, vvar->clock.shift);
 	} while (unlikely(vvar_read_retry(vvar, seq)));
 
 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
@@ -174,7 +194,7 @@ notrace static __always_inline int do_realtime_stick(struct vvar_data *vvar,
 		ts->tv_sec = vvar->wall_time_sec;
 		ns = vvar->wall_time_snsec;
 		ns += vgetsns_stick(vvar);
-		ns >>= vvar->clock.shift;
+		ns = __shr64(ns, vvar->clock.shift);
 	} while (unlikely(vvar_read_retry(vvar, seq)));
 
 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
@@ -194,7 +214,7 @@ notrace static __always_inline int do_monotonic(struct vvar_data *vvar,
 		ts->tv_sec = vvar->monotonic_time_sec;
 		ns = vvar->monotonic_time_snsec;
 		ns += vgetsns(vvar);
-		ns >>= vvar->clock.shift;
+		ns = __shr64(ns, vvar->clock.shift);
 	} while (unlikely(vvar_read_retry(vvar, seq)));
 
 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
@@ -214,7 +234,7 @@ notrace static __always_inline int do_monotonic_stick(struct vvar_data *vvar,
 		ts->tv_sec = vvar->monotonic_time_sec;
 		ns = vvar->monotonic_time_snsec;
 		ns += vgetsns_stick(vvar);
-		ns >>= vvar->clock.shift;
+		ns = __shr64(ns, vvar->clock.shift);
 	} while (unlikely(vvar_read_retry(vvar, seq)));
 
 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);

---
base-commit: defaf1a2113a22b00dfa1abc0fd2014820eaf065
change-id: 20240717-sparc-shr64-2f00a7884770

Best regards,
-- 
Koakuma <koachan@...tonmail.com>