[PATCH 1/1] riscv: __asm_copy_to-from_user: Improve using word copy if size < 9*SZREG

[Palmer Dabbelt]

On Fri, 30 Jul 2021 06:52:44 PDT (-0700), akira.tsukamoto at gmail.com wrote:
> Reduce the number of slow byte_copy when the size is in between
> 2*SZREG to 9*SZREG by using none unrolled word_copy.
>
> Without it any size smaller than 9*SZREG will be using slow byte_copy
> instead of none unrolled word_copy.
>
> Signed-off-by: Akira Tsukamoto <akira.tsukamoto at gmail.com>
> ---
>  arch/riscv/lib/uaccess.S | 46 ++++++++++++++++++++++++++++++++++++----
>  1 file changed, 42 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
> index 63bc691cff91..6a80d5517afc 100644
> --- a/arch/riscv/lib/uaccess.S
> +++ b/arch/riscv/lib/uaccess.S
> @@ -34,8 +34,10 @@ ENTRY(__asm_copy_from_user)
>  	/*
>  	 * Use byte copy only if too small.
>  	 * SZREG holds 4 for RV32 and 8 for RV64
> +	 * a3 - 2*SZREG is minimum size for word_copy
> +	 *      1*SZREG for aligning dst + 1*SZREG for word_copy
>  	 */
> -	li	a3, 9*SZREG /* size must be larger than size in word_copy */
> +	li	a3, 2*SZREG
>  	bltu	a2, a3, .Lbyte_copy_tail
>
>  	/*
> @@ -66,9 +68,40 @@ ENTRY(__asm_copy_from_user)
>  	andi	a3, a1, SZREG-1
>  	bnez	a3, .Lshift_copy
>
> +.Lcheck_size_bulk:
> +	/*
> +	 * Evaluate the size if possible to use unrolled.
> +	 * The word_copy_unlrolled requires larger than 8*SZREG
> +	 */
> +	li	a3, 8*SZREG
> +	add	a4, a0, a3
> +	bltu	a4, t0, .Lword_copy_unlrolled
> +
>  .Lword_copy:
> -        /*
> -	 * Both src and dst are aligned, unrolled word copy
> +	/*
> +	 * Both src and dst are aligned
> +	 * None unrolled word copy with every 1*SZREG iteration
> +	 *
> +	 * a0 - start of aligned dst
> +	 * a1 - start of aligned src
> +	 * t0 - end of aligned dst
> +	 */
> +	bgeu	a0, t0, .Lbyte_copy_tail /* check if end of copy */
> +	addi	t0, t0, -(SZREG) /* not to over run */
> +1:
> +	REG_L	a5, 0(a1)
> +	addi	a1, a1, SZREG
> +	REG_S	a5, 0(a0)
> +	addi	a0, a0, SZREG
> +	bltu	a0, t0, 1b
> +
> +	addi	t0, t0, SZREG /* revert to original value */
> +	j	.Lbyte_copy_tail
> +
> +.Lword_copy_unlrolled:
> +	/*
> +	 * Both src and dst are aligned
> +	 * Unrolled word copy with every 8*SZREG iteration
>  	 *
>  	 * a0 - start of aligned dst
>  	 * a1 - start of aligned src
> @@ -97,7 +130,12 @@ ENTRY(__asm_copy_from_user)
>  	bltu	a0, t0, 2b
>
>  	addi	t0, t0, 8*SZREG /* revert to original value */
> -	j	.Lbyte_copy_tail
> +
> +	/*
> +	 * Remaining might large enough for word_copy to reduce slow byte
> +	 * copy
> +	 */
> +	j	.Lcheck_size_bulk
>
>  .Lshift_copy:

I'm still not convinced that going all the way to such a large unrolling 
factor is a net win, but this at least provides a much smoother cost 
curve.

That said, this is causing my 32-bit configs to hang.  There were a few 
conflicts so I may have messed something up, but nothing is jumping out 
at me.  I've put what I ended up with on a branch, if you have time to 
look that'd be great but if not then I'll take another shot at this when 
I get back around to it.

    https://git.kernel.org/pub/scm/linux/kernel/git/palmer/linux.git/commit/?h=wip-word_user_copy

Here's the backtrace, though that's probably not all that useful:

[    0.703694] Unable to handle kernel NULL pointer dereference at virtual address 000005a8
[    0.704194] Oops [#1]
[    0.704301] Modules linked in:[    0.704463] CPU: 2 PID: 1 Comm: init Not tainted 5.14.0-rc1-00016-g59461ddb9dbd #5
[    0.704660] Hardware name: riscv-virtio,qemu (DT)
[    0.704802] epc : walk_stackframe+0xac/0xc2[    0.704941]  ra : dump_backtrace+0x1a/0x22
[    0.705074] epc : c0004558 ra : c0004588 sp : c1c5fe10
[    0.705216]  gp : c18b41c8 tp : c1cd8000 t0 : 00000000[    0.705357]  t1 : ffffffff t2 : 00000000 s0 : c1c5fe40
[    0.705506]  s1 : c11313dc a0 : 00000000 a1 : 00000000
[    0.705647]  a2 : c06fd2c2 a3 : c11313dc a4 : c084292d[    0.705787]  a5 : 00000000 a6 : c1864cb8 a7 : 3fffffff
[    0.705926]  s2 : 00000000 s3 : c1123e88 s4 : 00000000
[    0.706066]  s5 : c11313dc s6 : c06fd2c2 s7 : 00000001[    0.706206]  s8 : 00000000 s9 : 95af6e28 s10: 00000000
[    0.706345]  s11: 00000001 t3 : 00000000 t4 : 00000000
[    0.706482]  t5 : 00000001 t6 : 00000000[    0.706594] status: 00000100 badaddr: 000005a8 cause: 0000000d
[    0.706809] [<c0004558>] walk_stackframe+0xac/0xc2
[    0.707019] [<c0004588>] dump_backtrace+0x1a/0x22[    0.707149] [<c06fd312>] show_stack+0x2c/0x38
[    0.707271] [<c06ffba4>] dump_stack_lvl+0x40/0x58
[    0.707400] [<c06ffbce>] dump_stack+0x12/0x1a[    0.707521] [<c06fd4f6>] panic+0xfa/0x2a6
[    0.707632] [<c000e2f4>] do_exit+0x7a8/0x7ac
[    0.707749] [<c000eefa>] do_group_exit+0x2a/0x7e[    0.707872] [<c000ef60>] __wake_up_parent+0x0/0x20
[    0.707999] [<c0003020>] ret_from_syscall+0x0/0x2
[    0.708385] ---[ end trace 260976561a3770d1 ]---