linux-kernel - [PATCH] powerpc/8xx: Remove offset in SPRN_M

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Message-ID: <9710d960b512996e64beebfd368cfeaadb28b3ba.1755509047.git.christophe.leroy@csgroup.eu>
Date: Mon, 18 Aug 2025 11:24:22 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Naveen N Rao <naveen@...nel.org>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH] powerpc/8xx: Remove offset in SPRN_M_TWB

SPRN_M_TWB contains the address of task PGD minus an offset which
compensates the offset required when accessing the kernel PGDIR.
However, since commit ac9f97ff8b32 ("powerpc/8xx: Inconditionally use
task PGDIR in DTLB misses") and commit 33c527522f39 ("powerpc/8xx:
Inconditionally use task PGDIR in ITLB misses") kernel PGDIR is not
used anymore in hot pathes.

Remove this offset which was added by
commit fde5a9057fcf ("powerpc/8xx: Optimise access to swapper_pg_dir")

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/kernel/head_8xx.S       | 16 +++++++++-------
 arch/powerpc/mm/nohash/mmu_context.c | 10 +---------
 2 files changed, 10 insertions(+), 16 deletions(-)

diff --git a/arch/powerpc/kernel/head_8xx.S b/arch/powerpc/kernel/head_8xx.S
index 56c5ebe21b99..78942fd6b4b9 100644
--- a/arch/powerpc/kernel/head_8xx.S
+++ b/arch/powerpc/kernel/head_8xx.S
@@ -190,7 +190,7 @@ instruction_counter:
 	INVALIDATE_ADJACENT_PAGES_CPU15(r10, r11)
 	mtspr	SPRN_MD_EPN, r10
 	mfspr	r10, SPRN_M_TWB	/* Get level 1 table */
-	lwz	r11, (swapper_pg_dir-PAGE_OFFSET)@l(r10)	/* Get level 1 entry */
+	lwz	r11, 0(r10)	/* Get level 1 entry */
 	mtspr	SPRN_MD_TWC, r11
 	mfspr	r10, SPRN_MD_TWC
 	lwz	r10, 0(r10)	/* Get the pte */
@@ -233,7 +233,7 @@ instruction_counter:
 	 */
 	mfspr	r10, SPRN_MD_EPN
 	mfspr	r10, SPRN_M_TWB	/* Get level 1 table */
-	lwz	r11, (swapper_pg_dir-PAGE_OFFSET)@l(r10)	/* Get level 1 entry */
+	lwz	r11, 0(r10)	/* Get level 1 entry */
 
 	mtspr	SPRN_MD_TWC, r11
 	mfspr	r10, SPRN_MD_TWC
@@ -375,7 +375,7 @@ FixupPGD:
 	mfspr	r10, SPRN_DAR
 	mtspr	SPRN_MD_EPN, r10
 	mfspr	r11, SPRN_M_TWB	/* Get level 1 table */
-	lwz	r10, (swapper_pg_dir - PAGE_OFFSET)@l(r11)	/* Get the level 1 entry */
+	lwz	r10, 0(r11)	/* Get the level 1 entry */
 	cmpwi	cr1, r10, 0
 	bne	cr1, 1f
 
@@ -384,7 +384,7 @@ FixupPGD:
 	lwz	r10, (swapper_pg_dir - PAGE_OFFSET)@l(r10)	/* Get the level 1 entry */
 	cmpwi	cr1, r10, 0
 	beq	cr1, 1f
-	stw	r10, (swapper_pg_dir - PAGE_OFFSET)@l(r11)	/* Set the level 1 entry */
+	stw	r10, 0(r11)	/* Set the level 1 entry */
 	mfspr	r10, SPRN_M_TW
 	mtcr	r10
 	mfspr	r10, SPRN_SPRG_SCRATCH0
@@ -412,9 +412,10 @@ FixupDAR:/* Entry point for dcbx workaround. */
 	tophys(r11, r10)
 	mfspr	r11, SPRN_M_TWB	/* Get level 1 table */
 	rlwinm	r11, r11, 0, 20, 31
-	oris	r11, r11, (swapper_pg_dir - PAGE_OFFSET)@ha
+	oris	r11, r11, (swapper_pg_dir - PAGE_OFFSET)@h
+	ori	r11, r11, (swapper_pg_dir - PAGE_OFFSET)@l
 3:
-	lwz	r11, (swapper_pg_dir-PAGE_OFFSET)@l(r11)	/* Get the level 1 entry */
+	lwz	r11, 0(r11)	/* Get the level 1 entry */
 	rlwinm	r11, r11, 0, ~_PMD_PAGE_8M
 	mtspr	SPRN_MD_TWC, r11
 	mfspr	r11, SPRN_MD_TWC
@@ -535,7 +536,8 @@ start_here:
 	li	r0,0
 	stwu	r0,THREAD_SIZE-STACK_FRAME_MIN_SIZE(r1)
 
-	lis	r6, swapper_pg_dir@ha
+	lis	r6, swapper_pg_dir@h
+	ori	r6, r6, swapper_pg_dir@l
 	tophys(r6,r6)
 	mtspr	SPRN_M_TWB, r6
 
diff --git a/arch/powerpc/mm/nohash/mmu_context.c b/arch/powerpc/mm/nohash/mmu_context.c
index a1a4e697251a..28a96a10c907 100644
--- a/arch/powerpc/mm/nohash/mmu_context.c
+++ b/arch/powerpc/mm/nohash/mmu_context.c
@@ -203,15 +203,7 @@ static unsigned int steal_context_up(unsigned int id)
 static void set_context(unsigned long id, pgd_t *pgd)
 {
 	if (IS_ENABLED(CONFIG_PPC_8xx)) {
-		s16 offset = (s16)(__pa(swapper_pg_dir));
-
-		/*
-		 * Register M_TWB will contain base address of level 1 table minus the
-		 * lower part of the kernel PGDIR base address, so that all accesses to
-		 * level 1 table are done relative to lower part of kernel PGDIR base
-		 * address.
-		 */
-		mtspr(SPRN_M_TWB, __pa(pgd) - offset);
+		mtspr(SPRN_M_TWB, __pa(pgd));
 
 		/* Update context */
 		mtspr(SPRN_M_CASID, id - 1);
-- 
2.49.0