linux-kernel - [PATCH] MIPS: Take in account load hazards for HI/LO restoring

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Message-ID: <20231012162027.3411684-1-lis8215@gmail.com>
Date:   Thu, 12 Oct 2023 19:20:27 +0300
From:   Siarhei Volkau <lis8215@...il.com>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Siarhei Volkau <lis8215@...il.com>
Cc:     Paul Cercueil <paul@...pouillou.net>
Subject: [PATCH] MIPS: Take in account load hazards for HI/LO restoring

MIPS CPUs usually have 1 to 4 cycles load hazards, thus doing load
and right after move to HI/LO will usually stall the pipeline for
significant amount of time. Let's take it into account and separate
loads and mthi/lo in instruction sequence.

The patch uses t6 and t7 registers as temporaries in addition to t8.

The patch tries to deal with SmartMIPS, but I know little about and
haven't tested it.

Signed-off-by: Siarhei Volkau <lis8215@...il.com>
---
 arch/mips/include/asm/stackframe.h | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/arch/mips/include/asm/stackframe.h b/arch/mips/include/asm/stackframe.h
index a8705aef47e1..3821d91b00fd 100644
--- a/arch/mips/include/asm/stackframe.h
+++ b/arch/mips/include/asm/stackframe.h
@@ -308,17 +308,11 @@
 		jal	octeon_mult_restore
 #endif
 #ifdef CONFIG_CPU_HAS_SMARTMIPS
-		LONG_L	$24, PT_ACX(sp)
-		mtlhx	$24
-		LONG_L	$24, PT_HI(sp)
-		mtlhx	$24
-		LONG_L	$24, PT_LO(sp)
-		mtlhx	$24
-#elif !defined(CONFIG_CPU_MIPSR6)
+		LONG_L	$14, PT_ACX(sp)
+#endif
+#if defined(CONFIG_CPU_HAS_SMARTMIPS) || !defined(CONFIG_CPU_MIPSR6)
 		LONG_L	$24, PT_LO(sp)
-		mtlo	$24
-		LONG_L	$24, PT_HI(sp)
-		mthi	$24
+		LONG_L	$15, PT_HI(sp)
 #endif
 #ifdef CONFIG_32BIT
 		cfi_ld	$8, PT_R8, \docfi
@@ -327,6 +321,14 @@
 		cfi_ld	$10, PT_R10, \docfi
 		cfi_ld	$11, PT_R11, \docfi
 		cfi_ld	$12, PT_R12, \docfi
+#ifdef CONFIG_CPU_HAS_SMARTMIPS
+		mtlhx	$14
+		mtlhx	$15
+		mtlhx	$24
+#elif !defined(CONFIG_CPU_MIPSR6)
+		mtlo	$24
+		mthi	$15
+#endif
 		cfi_ld	$13, PT_R13, \docfi
 		cfi_ld	$14, PT_R14, \docfi
 		cfi_ld	$15, PT_R15, \docfi
-- 
2.41.0