linux-kernel - [PATCH RFC v2 18/28] ARM: entry: Drop the superfast ret_fast

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Message-Id: <20241029-arm-generic-entry-v2-18-573519abef38@linaro.org>
Date: Tue, 29 Oct 2024 11:52:58 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Oleg Nesterov <oleg@...hat.com>, Russell King <linux@...linux.org.uk>, 
 Kees Cook <kees@...nel.org>, Andy Lutomirski <luto@...capital.net>, 
 Will Drewry <wad@...omium.org>, Frederic Weisbecker <frederic@...nel.org>, 
 "Paul E. McKenney" <paulmck@...nel.org>, 
 Jinjie Ruan <ruanjinjie@...wei.com>, Arnd Bergmann <arnd@...db.de>, 
 Ard Biesheuvel <ardb@...nel.org>, Al Viro <viro@...iv.linux.org.uk>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 Linus Walleij <linus.walleij@...aro.org>
Subject: [PATCH RFC v2 18/28] ARM: entry: Drop the superfast
 ret_fast_syscall

When not using any irqflags tracing, context tracking or rseq
debugging, the ARM kernel uses a "superfast" ret_fast_syscall
with a very few assembly instructions.

With generic entry, such local optimizations are not really
possible, we need to call C unconditionally and take the
(hopefully small) overhead.

With generic entry in C, most of the tracing (etc) calls are
replaced by stubs and optimized out by the compiler.

Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
---
 arch/arm/kernel/entry-common.S | 29 +----------------------------
 1 file changed, 1 insertion(+), 28 deletions(-)

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index 3559e0a9f5e0..dddbd6ba1eb9 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -30,34 +30,8 @@ saved_pc	.req	lr

 	.section .entry.text,"ax",%progbits
 	.align	5
-#if !(IS_ENABLED(CONFIG_TRACE_IRQFLAGS) || IS_ENABLED(CONFIG_CONTEXT_TRACKING_USER) || \
-	IS_ENABLED(CONFIG_DEBUG_RSEQ))
 /*
- * This is the fast syscall return path.  We do as little as possible here,
- * such as avoiding writing r0 to the stack.  We only use this path if we
- * have tracing, context tracking and rseq debug disabled - the overheads
- * from those features make this path too inefficient.
- */
-ret_fast_syscall:
- UNWIND(.fnstart	)
- UNWIND(.cantunwind	)
-	disable_irq_notrace			@ disable interrupts
-	ldr	r1, [tsk, #TI_FLAGS]		@ re-check for syscall tracing
-	movs	r1, r1, lsl #16
-	bne	fast_work_pending
-
-	restore_user_regs fast = 0, offset = S_OFF
- UNWIND(.fnend		)
-ENDPROC(ret_fast_syscall)
-
-	/* Ok, we need to do extra processing, enter the slow path. */
-fast_work_pending:
-	add	sp, sp, #(S_R0 + S_OFF)
-	/* fall through to work_pending */
-#else
-/*
- * The "replacement" ret_fast_syscall for when tracing, context tracking,
- * or rseq debug is enabled.  As we will need to call out to some C functions,
+ * As we will need to call out to some C functions,
  * we save r0 first to avoid needing to save registers around each C function
  * call.
  */
@@ -75,7 +49,6 @@ ret_fast_syscall:
 ENDPROC(ret_fast_syscall)

 	/* Slower path - fall through to work_pending */
-#endif

 	tst	r1, #_TIF_SYSCALL_WORK
 	beq	slow_work_pending

-- 
2.46.2