[PATCH v2] MIPS: Take in account load hazards for HI/LO restoring

[PATCH v2] MIPS: Take in account load hazards for HI/LO restoring

[Siarhei Volkau]

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.

Changes in v2:
- clear separation of actions for SmartMIPS and pre-MIPSR6.

Signed-off-by: Siarhei Volkau <lis8215@gmail.com>
---
 arch/mips/include/asm/stackframe.h | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/arch/mips/include/asm/stackframe.h b/arch/mips/include/asm/stackframe.h
index a8705aef47e1..a13431379073 100644
--- a/arch/mips/include/asm/stackframe.h
+++ b/arch/mips/include/asm/stackframe.h
@@ -308,17 +308,12 @@
 		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	$14, PT_ACX(sp)
 		LONG_L	$24, PT_LO(sp)
-		mtlhx	$24
+		LONG_L	$15, PT_HI(sp)
 #elif !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 +322,14 @@
 		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
-- 
2.44.0

Re: [PATCH v2] MIPS: Take in account load hazards for HI/LO restoring

[Thomas Bogendoerfer]

On Tue, Apr 30, 2024 at 06:45:58PM +0300, Siarhei Volkau wrote:
> 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.
> 
> Changes in v2:
> - clear separation of actions for SmartMIPS and pre-MIPSR6.
> 
> Signed-off-by: Siarhei Volkau <lis8215@gmail.com>
> ---
>  arch/mips/include/asm/stackframe.h | 19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 

applied to mips-next.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]