Messages in this thread Patch in this message | | | Subject | Re: [PATCH v3 1/1] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd() | From | John Garry <> | Date | Tue, 7 Dec 2021 16:17:19 +0000 |
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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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