linux-kernel - Re: [PATCH v3 4/4] powerpc/inst: Optimise copy_inst_from_kernel

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Message-ID: <e1f310b4-2e23-0fa4-424d-271395824438@csgroup.eu>
Date:   Mon, 29 Nov 2021 08:22:22 +0100
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: Re: [PATCH v3 4/4] powerpc/inst: Optimise
 copy_inst_from_kernel_nofault()



Le 27/11/2021 à 19:10, Christophe Leroy a écrit :
> copy_inst_from_kernel_nofault() uses copy_from_kernel_nofault() to
> copy one or two 32bits words. This means calling an out-of-line
> function which itself calls back copy_from_kernel_nofault_allowed()
> then performs a generic copy with loops.
> 
> Rewrite copy_inst_from_kernel_nofault() to do everything at a
> single place and use __get_kernel_nofault() directly to perform
> single accesses without loops.
> 
> Before the patch:
> 
> 	00000018 <copy_inst_from_kernel_nofault>:
> 	  18:	94 21 ff e0 	stwu    r1,-32(r1)
> 	  1c:	7c 08 02 a6 	mflr    r0
> 	  20:	38 a0 00 04 	li      r5,4
> 	  24:	93 e1 00 1c 	stw     r31,28(r1)
> 	  28:	7c 7f 1b 78 	mr      r31,r3
> 	  2c:	38 61 00 08 	addi    r3,r1,8
> 	  30:	90 01 00 24 	stw     r0,36(r1)
> 	  34:	48 00 00 01 	bl      34 <copy_inst_from_kernel_nofault+0x1c>
> 				34: R_PPC_REL24	copy_from_kernel_nofault
> 	  38:	2c 03 00 00 	cmpwi   r3,0
> 	  3c:	40 82 00 0c 	bne     48 <copy_inst_from_kernel_nofault+0x30>
> 	  40:	81 21 00 08 	lwz     r9,8(r1)
> 	  44:	91 3f 00 00 	stw     r9,0(r31)
> 	  48:	80 01 00 24 	lwz     r0,36(r1)
> 	  4c:	83 e1 00 1c 	lwz     r31,28(r1)
> 	  50:	38 21 00 20 	addi    r1,r1,32
> 	  54:	7c 08 03 a6 	mtlr    r0
> 	  58:	4e 80 00 20 	blr
> 
> After the patch:
> 
> 	00000018 <copy_inst_from_kernel_nofault>:
> 	  18:	3d 20 b0 00 	lis     r9,-20480
> 	  1c:	7c 04 48 40 	cmplw   r4,r9
> 	  20:	7c 69 1b 78 	mr      r9,r3
> 	  24:	41 80 00 2c 	blt     50 <copy_inst_from_kernel_nofault+0x38>
> 	  28:	81 42 04 d0 	lwz     r10,1232(r2)
> 	  2c:	39 4a 00 01 	addi    r10,r10,1
> 	  30:	91 42 04 d0 	stw     r10,1232(r2)
> 	  34:	80 e4 00 00 	lwz     r7,0(r4)
> 	  38:	81 42 04 d0 	lwz     r10,1232(r2)
> 	  3c:	38 60 00 00 	li      r3,0
> 	  40:	39 4a ff ff 	addi    r10,r10,-1
> 	  44:	91 42 04 d0 	stw     r10,1232(r2)
> 	  48:	90 e9 00 00 	stw     r7,0(r9)
> 	  4c:	4e 80 00 20 	blr
> 
> 	  50:	38 60 ff de 	li      r3,-34
> 	  54:	4e 80 00 20 	blr
> 	  58:	81 22 04 d0 	lwz     r9,1232(r2)
> 	  5c:	38 60 ff f2 	li      r3,-14
> 	  60:	39 29 ff ff 	addi    r9,r9,-1
> 	  64:	91 22 04 d0 	stw     r9,1232(r2)
> 	  68:	4e 80 00 20 	blr
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
> v3: New
> ---
>   arch/powerpc/mm/maccess.c | 18 ++++++++++++------
>   1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/powerpc/mm/maccess.c b/arch/powerpc/mm/maccess.c
> index 5abae96b2b46..90309806f5eb 100644
> --- a/arch/powerpc/mm/maccess.c
> +++ b/arch/powerpc/mm/maccess.c
> @@ -15,16 +15,22 @@ bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
>   int copy_inst_from_kernel_nofault(ppc_inst_t *inst, u32 *src)
>   {
>   	unsigned int val, suffix;
> -	int err;
>   
> -	err = copy_from_kernel_nofault(&val, src, sizeof(val));
> -	if (err)
> -		return err;
> +	if (unlikely(!is_kernel_addr((unsigned long)src)))
> +		return -ERANGE;
> +
> +	pagefault_disable();

Allthough generic version of copy_from_kernel_nofault() does it, 
disabling pagefault is pointless here because we are accessing kernel 
addresses only, so a page fault will always fail via bad_kernel_fault(), 
it will never reach the faulthandler_disabled() test.

> +	__get_kernel_nofault(&val, src, u32, Efault);
>   	if (IS_ENABLED(CONFIG_PPC64) && get_op(val) == OP_PREFIX) {
> -		err = copy_from_kernel_nofault(&suffix, src + 1, sizeof(suffix));
> +		__get_kernel_nofault(&suffix, src + 1, u32, Efault);
> +		pagefault_enable();
>   		*inst = ppc_inst_prefix(val, suffix);
>   	} else {
> +		pagefault_enable();
>   		*inst = ppc_inst(val);
>   	}
> -	return err;
> +	return 0;
> +Efault:
> +	pagefault_enable();
> +	return -EFAULT;
>   }
>