linux-kernel - [PATCH] powerpc/32s: Speed up likely path of kuap_update

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Message-Id: <848f18d213b8341939add7302dc4ef80cc7a12e3.1620307636.git.christophe.leroy@csgroup.eu>
Date:   Thu,  6 May 2021 13:27:31 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH] powerpc/32s: Speed up likely path of kuap_update_sr()

In most cases, kuap_update_sr() will update a single segment
register.

We know that first update will always be done, if there is no
segment register to update at all, kuap_update_sr() is not
called.

Avoid recurring calculations and tests in that case.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/include/asm/book3s/32/kup.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
index 1670dfe9d4f1..c1f7c2e625a6 100644
--- a/arch/powerpc/include/asm/book3s/32/kup.h
+++ b/arch/powerpc/include/asm/book3s/32/kup.h
@@ -15,11 +15,13 @@ static inline void kuap_update_sr(u32 sr, u32 addr, u32 end)
 {
 	addr &= 0xf0000000;	/* align addr to start of segment */
 	barrier();	/* make sure thread.kuap is updated before playing with SRs */
-	while (addr < end) {
+	for (;;) {
 		mtsr(sr, addr);
+		addr += 0x10000000;	/* address of next segment */
+		if (addr >= end)
+			break;
 		sr += 0x111;		/* next VSID */
 		sr &= 0xf0ffffff;	/* clear VSID overflow */
-		addr += 0x10000000;	/* address of next segment */
 	}
 	isync();	/* Context sync required after mtsr() */
 }
-- 
2.25.0