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SubjectRe: [PATCH v3 1/1] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd()
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On 07/12/2021 09:41, Zhen Lei via iommu wrote:
> Although the parameter 'cmd' is always passed by a local array variable,
> and only this function modifies it, the compiler does not know this. Every
> time the 'cmd' variable is updated, a memory write operation is generated.
> This generates many useless instruction operations.

I'd hardly call them useless. More like inefficient or sub-optimum.

>
> To guide the compiler for proper optimization, 'cmd' is defined as a local
> array variable, and copied to the output parameter at a time when the
> function is returned.
>
> The optimization effect can be viewed by running the "size arm-smmu-v3.o"
> command.
>
> Before:
> text data bss dec hex
> 28246 1332 56 29634 73c2
>
> After:
> text data bss dec hex
> 28134 1332 56 29522 7352
>
> For example:
> cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);
> case CMDQ_OP_TLBI_EL2_VA:
> cmd[0] |= FIELD_PREP(CMDQ_TLBI_0_NUM, ent->tlbi.num);
> cmd[0] |= FIELD_PREP(CMDQ_TLBI_0_SCALE, ent->tlbi.scale);
> cmd[0] |= FIELD_PREP(CMDQ_TLBI_0_ASID, ent->tlbi.asid);
> cmd[1] |= FIELD_PREP(CMDQ_TLBI_1_LEAF, ent->tlbi.leaf);
> cmd[1] |= FIELD_PREP(CMDQ_TLBI_1_TTL, ent->tlbi.ttl);
> cmd[1] |= FIELD_PREP(CMDQ_TLBI_1_TG, ent->tlbi.tg);
> cmd[1] |= ent->tlbi.addr & CMDQ_TLBI_1_VA_MASK;
>
> Before:
> Each "cmd[0] |=" or "cmd[1] |=" operation generates a "str" instruction,
> sum = 8.
>
> ldrb w4, [x1, #8] //w4 = ent->tlbi.num
> ubfiz x4, x4, #12, #5
> mov w0, #0x0
> orr x4, x4, x3
> str x4, [x2]
> autiasp
> ldrb w3, [x1, #9] //w3 = ent->tlbi.scale
> ubfiz x3, x3, #20, #5
> orr x3, x3, x4
> str x3, [x2]
> ldrh w4, [x1, #10] //w4 = ent->tlbi.asid
> orr x3, x3, x4, lsl #48
> str x3, [x2]
> ldrb w3, [x1, #14] //w3 = ent->tlbi.leaf
> str x3, [x2, #8]
> ldrb w4, [x1, #15] //w4 = ent->tlbi.ttl
> ubfiz x4, x4, #8, #2
> orr x4, x4, x3
> str x4, [x2, #8]
> ldrb w3, [x1, #16] //ent->tlbi.tg
> ubfiz x3, x3, #10, #2
> orr x3, x3, x4
> str x3, [x2, #8]
> ldr x1, [x1, #24] //ent->tlbi.addr
> and x1, x1, #0xfffffffffffff000
> orr x1, x1, x3
> str x1, [x2, #8]
> ret
>
> After:
> All "cmd[0] |=" and "cmd[1] |=" operations generate a "stp" instruction,
> sum = 1.
>
> 3e8:
> mov w0, #0x0
> autiasp
> stp x2, x1, [x3]
> ret
> bti j
> 3fc:
> ldrb w5, [x1, #8] //w5 = ent->tlbi.num
> mov x2, #0x22 //x2 = ent->opcode = CMDQ_0_OP
> ldrb w6, [x1, #9] //w6 = ent->tlbi.scale
> ubfiz x5, x5, #12, #5
> ldrb w0, [x1, #16] //w0 = ent->tlbi.tg
> orr x5, x5, x2
> ldrb w7, [x1, #15] //w7 = ent->tlbi.ttl
> ldr x4, [x1, #24] //x4 = ent->tlbi.addr
> ubfiz x0, x0, #10, #2
> ldrh w2, [x1, #10] //w2 = ent->tlbi.asid
> ubfiz x6, x6, #20, #5
> ldrb w8, [x1, #14] //w8 = ent->tlbi.leaf
> and x4, x4, #0xfffffffffffff000
> ubfiz x1, x7, #8, #2
> orr x1, x0, x1
> orr x2, x6, x2, lsl #48
> orr x0, x4, x8
> orr x2, x2, x5
> orr x1, x1, x0
> b 3e8
>
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
> drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index f5848b351b19359..e55dfc14cac6005 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -234,10 +234,12 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent)
> }
>
> /* High-level queue accessors */
> -static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
> +static int arm_smmu_cmdq_build_cmd(u64 *out_cmd, struct arm_smmu_cmdq_ent *ent)
> {
> - memset(cmd, 0, 1 << CMDQ_ENT_SZ_SHIFT);
> - cmd[0] |= FIELD_PREP(CMDQ_0_OP, ent->opcode);
> + int i;
> + u64 cmd[CMDQ_ENT_DWORDS] = {0};

I thought that just {} was preferred. Or could have:
u64 cmd[CMDQ_ENT_DWORDS] = {FIELD_PREP(CMDQ_0_OP, ent->opcode), };
to be more concise

> +
> + cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);
>
> switch (ent->opcode) {
> case CMDQ_OP_TLBI_EL2_ALL:
> @@ -332,6 +334,9 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
> return -ENOENT;
> }
>
> + for (i = 0; i < CMDQ_ENT_DWORDS; i++)
> + out_cmd[i] = cmd[i];

how about memcpy() instead?

> +
> return 0;
> }

Did you notice any performance change with this change?

Thanks,
John

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