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Subject: Re: [RFC PATCH v1 14/16] powerpc/32: implement fast entry for
 syscalls on non BOOKE
From:   christophe leroy <christophe.leroy@c-s.fr>
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Nicholas Piggin <npiggin@gmail.com>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
References: <cover.1549630193.git.christophe.leroy@c-s.fr>
 <ba23b45ec5e4a0acb040cacc48949f7bc984b0ee.1549630193.git.christophe.leroy@c-s.fr>
Message-ID: <9050ee8b-2f92-aff0-56a2-8e5d43d3c593@c-s.fr>
Date:   Sun, 10 Feb 2019 19:05:35 +0100
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Le 08/02/2019 à 13:52, Christophe Leroy a écrit :
> This patch implements a fast entry for syscalls.
> 
> Syscalls don't have to preserve non volatile registers except LR.
> 
> This patch then implement a fast entry for syscalls, where
> volatile registers get clobbered.
> 
> As this entry is dedicated to syscall it always sets MSR_EE
> and warns in case MSR_EE was previously off
> 
> It also assumes that the call is always from user, system calls are
> unexpected from kernel.
> 
> The overall series improves null_syscall selftest by 12,5% on an 83xx
> and by 17% on a 8xx.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>

The following addition is required to avoid build failure on 40x as 
global_dbcr0 is going to be accessed from head_40x.S

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 85f1fc88c237..987f0fafc999 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S
@@ -1247,6 +1247,7 @@ load_dbcr0:

  	.section .bss
  	.align	4
+	.global global_dbcr0
  global_dbcr0:
  	.space	8*NR_CPUS
  	.previous

Christophe

> ---
>   arch/powerpc/kernel/entry_32.S | 42 +++++++++++++++++++++
>   arch/powerpc/kernel/head_32.S  |  3 +-
>   arch/powerpc/kernel/head_32.h  | 85 ++++++++++++++++++++++++++++++++++++++++--
>   arch/powerpc/kernel/head_40x.S |  3 +-
>   arch/powerpc/kernel/head_8xx.S |  3 +-
>   5 files changed, 126 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
> index 1e11528d45ae..137bd2103051 100644
> --- a/arch/powerpc/kernel/entry_32.S
> +++ b/arch/powerpc/kernel/entry_32.S
> @@ -335,6 +335,46 @@ stack_ovf:
>   	SYNC
>   	RFI
>   
> +#ifndef CONFIG_BOOKE	/* to be removed once BOOKE uses fast syscall entry */
> +#ifdef CONFIG_TRACE_IRQFLAGS
> +trace_syscall_entry_irq_off:
> +	/*
> +	 * The trace_hardirqs_off will use CALLER_ADDR0 and CALLER_ADDR1.
> +	 * If from user mode there is only one stack frame on the stack, and
> +	 * accessing CALLER_ADDR1 will cause oops. So we need create a dummy
> +	 * stack frame to make trace_hardirqs_on happy.
> +	 *
> +	 */
> +	stwu	r1,-32(r1)
> +
> +	/*
> +	 * Syscall shouldn't happen while interrupts are disabled,
> +	 * so let's do a warning here.
> +	 */
> +0:	trap
> +	EMIT_BUG_ENTRY 0b,__FILE__,__LINE__, BUGFLAG_WARNING
> +	bl	trace_hardirqs_on
> +
> +	addi	r1,r1,32
> +
> +	/* Now enable for real */
> +	LOAD_MSR_KERNEL(r10, MSR_KERNEL | MSR_EE)
> +	mtmsr	r10
> +
> +	REST_GPR(0, r1)
> +	REST_4GPRS(3, r1)
> +	REST_2GPRS(7, r1)
> +	b	DoSyscall
> +#endif /* CONFIG_TRACE_IRQFLAGS */
> +
> +	.globl	transfer_to_syscall
> +transfer_to_syscall:
> +#ifdef CONFIG_TRACE_IRQFLAGS
> +	andi.	r12,r9,MSR_EE
> +	beq-	trace_syscall_entry_irq_off
> +#endif /* CONFIG_TRACE_IRQFLAGS */
> +#endif /* !CONFIG_BOOKE */
> +
>   /*
>    * Handle a system call.
>    */
> @@ -346,9 +386,11 @@ _GLOBAL(DoSyscall)
>   	stw	r3,ORIG_GPR3(r1)
>   	li	r12,0
>   	stw	r12,RESULT(r1)
> +#ifdef CONFIG_BOOKE	/* to be removed once BOOKE uses fast syscall entry */
>   	lwz	r11,_CCR(r1)	/* Clear SO bit in CR */
>   	rlwinm	r11,r11,0,4,2
>   	stw	r11,_CCR(r1)
> +#endif
>   
>   #ifdef CONFIG_TRACE_IRQFLAGS
>   	/* Make sure interrupts are enabled */
> diff --git a/arch/powerpc/kernel/head_32.S b/arch/powerpc/kernel/head_32.S
> index 3a1df9edf6da..7576e1374a69 100644
> --- a/arch/powerpc/kernel/head_32.S
> +++ b/arch/powerpc/kernel/head_32.S
> @@ -368,8 +368,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_FPU_UNAVAILABLE)
>   	. = 0xc00
>   	DO_KVM  0xc00
>   SystemCall:
> -	EXCEPTION_PROLOG
> -	EXC_XFER_SYS(0xc00, DoSyscall)
> +	SYSCALL_ENTRY	0xc00
>   
>   /* Single step - not used on 601 */
>   	EXCEPTION(0xd00, SingleStep, single_step_exception, EXC_XFER_STD)
> diff --git a/arch/powerpc/kernel/head_32.h b/arch/powerpc/kernel/head_32.h
> index 14cb0af2f494..4a692553651f 100644
> --- a/arch/powerpc/kernel/head_32.h
> +++ b/arch/powerpc/kernel/head_32.h
> @@ -73,6 +73,87 @@
>   	SAVE_2GPRS(7, r11)
>   .endm
>   
> +.macro SYSCALL_ENTRY trapno
> +	mfspr	r12,SPRN_SPRG_THREAD
> +	mfcr	r10
> +	lwz	r11,TASK_STACK-THREAD(r12)
> +	mflr	r9
> +	addi	r11,r11,THREAD_SIZE - INT_FRAME_SIZE
> +	rlwinm	r10,r10,0,4,2	/* Clear SO bit in CR */
> +	tophys(r11,r11)
> +	stw	r10,_CCR(r11)		/* save registers */
> +	mfspr	r10,SPRN_SRR0
> +	stw	r9,_LINK(r11)
> +	mfspr	r9,SPRN_SRR1
> +	stw	r1,GPR1(r11)
> +	stw	r1,0(r11)
> +	tovirt(r1,r11)			/* set new kernel sp */
> +	stw	r10,_NIP(r11)
> +#ifdef CONFIG_40x
> +	rlwinm	r9,r9,0,14,12		/* clear MSR_WE (necessary?) */
> +#else
> +	LOAD_MSR_KERNEL(r10, MSR_KERNEL & ~(MSR_IR|MSR_DR)) /* can take exceptions */
> +	MTMSRD(r10)			/* (except for mach check in rtas) */
> +#endif
> +	lis	r10,STACK_FRAME_REGS_MARKER@ha /* exception frame marker */
> +	stw	r2,GPR2(r11)
> +	addi	r10,r10,STACK_FRAME_REGS_MARKER@l
> +	stw	r9,_MSR(r11)
> +	li	r2, \trapno + 1
> +	stw	r10,8(r11)
> +	stw	r2,_TRAP(r11)
> +	SAVE_GPR(0, r11)
> +	SAVE_4GPRS(3, r11)
> +	SAVE_2GPRS(7, r11)
> +	addi	r11,r1,STACK_FRAME_OVERHEAD
> +	addi	r2,r12,-THREAD
> +	stw	r11,PT_REGS(r12)
> +#if defined(CONFIG_40x)
> +	/* Check to see if the dbcr0 register is set up to debug.  Use the
> +	   internal debug mode bit to do this. */
> +	lwz	r12,THREAD_DBCR0(r12)
> +	andis.	r12,r12,DBCR0_IDM@h
> +#endif
> +	ACCOUNT_CPU_USER_ENTRY(r2, r11, r12)
> +#if defined(CONFIG_40x)
> +	beq+	3f
> +	/* From user and task is ptraced - load up global dbcr0 */
> +	li	r12,-1			/* clear all pending debug events */
> +	mtspr	SPRN_DBSR,r12
> +	lis	r11,global_dbcr0@ha
> +	tophys(r11,r11)
> +	addi	r11,r11,global_dbcr0@l
> +	lwz	r12,0(r11)
> +	mtspr	SPRN_DBCR0,r12
> +	lwz	r12,4(r11)
> +	addi	r12,r12,-1
> +	stw	r12,4(r11)
> +#endif
> +
> +3:
> +	tovirt(r2, r2)			/* set r2 to current */
> +	lis	r11, transfer_to_syscall@h
> +	ori	r11, r11, transfer_to_syscall@l
> +#ifdef CONFIG_TRACE_IRQFLAGS
> +	/*
> +	 * If MSR is changing we need to keep interrupts disabled at this point
> +	 * otherwise we might risk taking an interrupt before we tell lockdep
> +	 * they are enabled.
> +	 */
> +	LOAD_MSR_KERNEL(r10, MSR_KERNEL)
> +	rlwimi	r10, r9, 0, MSR_EE
> +#else
> +	LOAD_MSR_KERNEL(r10, MSR_KERNEL | MSR_EE)
> +#endif
> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PERF_EVENTS)
> +	mtspr	SPRN_NRI, r0
> +#endif
> +	mtspr	SPRN_SRR1,r10
> +	mtspr	SPRN_SRR0,r11
> +	SYNC
> +	RFI				/* jump to handler, enable MMU */
> +.endm
> +
>   /*
>    * Note: code which follows this uses cr0.eq (set if from kernel),
>    * r11, r12 (SRR0), and r9 (SRR1).
> @@ -119,8 +200,4 @@
>   	EXC_XFER_TEMPLATE(hdlr, n+1, MSR_KERNEL, transfer_to_handler, \
>   			  ret_from_except)
>   
> -#define EXC_XFER_SYS(n, hdlr)		\
> -	EXC_XFER_TEMPLATE(hdlr, n+1, MSR_KERNEL | MSR_EE, transfer_to_handler, \
> -			  ret_from_except)
> -
>   #endif /* __HEAD_32_H__ */
> diff --git a/arch/powerpc/kernel/head_40x.S b/arch/powerpc/kernel/head_40x.S
> index 1afab9190147..f5704488c4fc 100644
> --- a/arch/powerpc/kernel/head_40x.S
> +++ b/arch/powerpc/kernel/head_40x.S
> @@ -348,8 +348,7 @@ _ENTRY(saved_ksp_limit)
>   
>   /* 0x0C00 - System Call Exception */
>   	START_EXCEPTION(0x0C00,	SystemCall)
> -	EXCEPTION_PROLOG
> -	EXC_XFER_SYS(0xc00, DoSyscall)
> +	SYSCALL_ENTRY	0xc00
>   
>   	EXCEPTION(0x0D00, Trap_0D, unknown_exception, EXC_XFER_STD)
>   	EXCEPTION(0x0E00, Trap_0E, unknown_exception, EXC_XFER_STD)
> diff --git a/arch/powerpc/kernel/head_8xx.S b/arch/powerpc/kernel/head_8xx.S
> index 336b9921c0da..362b2150a3a0 100644
> --- a/arch/powerpc/kernel/head_8xx.S
> +++ b/arch/powerpc/kernel/head_8xx.S
> @@ -185,8 +185,7 @@ Alignment:
>   /* System call */
>   	. = 0xc00
>   SystemCall:
> -	EXCEPTION_PROLOG
> -	EXC_XFER_SYS(0xc00, DoSyscall)
> +	SYSCALL_ENTRY	0xc00
>   
>   /* Single step - not used on 601 */
>   	EXCEPTION(0xd00, SingleStep, single_step_exception, EXC_XFER_STD)
> 

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