linux-kernel - Re: [PATCH v3 1/1] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build

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Date:   Wed, 8 Dec 2021 21:40:10 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     John Garry <john.garry@...wei.com>, Will Deacon <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/1] iommu/arm-smmu-v3: Simplify useless instructions
 in arm_smmu_cmdq_build_cmd()



On 2021/12/8 0:17, John Garry wrote:
> 
>> +
>>       return 0;
>>   }
> 
> Did you notice any performance change with this change?

Hi John:
  Thanks for the tip. I wrote a test case today, and I found that the
performance did not go up but down. It's so weird. So I decided not to
change it, because it's also poorly readable. So I plan to make only
the following modifications:
@@ -237,7 +237,7 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent)
 static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
 {
        memset(cmd, 0, 1 << CMDQ_ENT_SZ_SHIFT);
-       cmd[0] |= FIELD_PREP(CMDQ_0_OP, ent->opcode);
+       cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);

        switch (ent->opcode) {
        case CMDQ_OP_TLBI_EL2_ALL:

This prevents the compiler from generating the following two inefficient
instructions:
     394:       f9400002        ldr     x2, [x0]	//x2 = cmd[0]
     398:       aa020062        orr     x2, x3, x2	//x3 = FIELD_PREP(CMDQ_0_OP, ent->opcode)

Maybe it's not worth changing because I've only seen a 0.x nanosecond reduction
in performance. But one thing is, it only comes with benefits, no side effects.

> 
> Thanks,
> John