linux-kernel - [PATCH v6 18/23] powerpc/e500: Don't pre-check write access on data TLB error

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Date: Mon, 24 Jun 2024 16:45:44 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Jason Gunthorpe <jgg@...dia.com>,
	Peter Xu <peterx@...hat.com>,
	Oscar Salvador <osalvador@...e.de>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH v6 18/23] powerpc/e500: Don't pre-check write access on data TLB error

Don't pre-check write access on read-only pages on data TLB error.

Load the TLB anyway and take a DSI exception when it happens. This
avoids reading SPRN_ESR at every data TLB error exception.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
v5: New
---
 arch/powerpc/kernel/head_85xx.S | 15 ---------------
 1 file changed, 15 deletions(-)

diff --git a/arch/powerpc/kernel/head_85xx.S b/arch/powerpc/kernel/head_85xx.S
index a305244afc9f..282e49c51deb 100644
--- a/arch/powerpc/kernel/head_85xx.S
+++ b/arch/powerpc/kernel/head_85xx.S
@@ -472,27 +472,12 @@ END_BTB_FLUSH_SECTION
 #endif
 
 4:
-	/* Mask of required permission bits. Note that while we
-	 * do copy ESR:ST to _PAGE_WRITE position as trying to write
-	 * to an RO page is pretty common, we don't do it with
-	 * _PAGE_DIRTY. We could do it, but it's a fairly rare
-	 * event so I'd rather take the overhead when it happens
-	 * rather than adding an instruction here. We should measure
-	 * whether the whole thing is worth it in the first place
-	 * as we could avoid loading SPRN_ESR completely in the first
-	 * place...
-	 *
-	 * TODO: Is it worth doing that mfspr & rlwimi in the first
-	 *       place or can we save a couple of instructions here ?
-	 */
-	mfspr	r12,SPRN_ESR
 #ifdef CONFIG_PTE_64BIT
 	li	r13,_PAGE_PRESENT|_PAGE_BAP_SR
 	oris	r13,r13,_PAGE_ACCESSED@h
 #else
 	li	r13,_PAGE_PRESENT|_PAGE_READ|_PAGE_ACCESSED
 #endif
-	rlwimi	r13,r12,11,29,29
 
 	FIND_PTE
 	andc.	r13,r13,r11		/* Check permission */
-- 
2.44.0