linux-kernel - Re: [PATCH 1/4] iommu/arm-smmu-v3: Use command queue batching helpers to improve performance

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Message-ID: <a3cdd5df-c028-5484-ce99-928a689d341a@huawei.com>
Date:   Mon, 16 Aug 2021 08:24:28 +0100
From:   John Garry <john.garry@...wei.com>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>, Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] iommu/arm-smmu-v3: Use command queue batching helpers
 to improve performance

> In addition, I find that function arm_smmu_cmdq_build_cmd() can also be optimized
> slightly, three useless instructions can be reduced.

I think that you could optimise further by pre-building commonly used 
commands.

For example, CMD_SYNC without MSI polling is always the same. And then 
only different in 1 field for MSI polling.

But you need to check if the performance gain is worth the change.

> 
> Case 1):
> void arm_smmu_cmdq_build_cmd_tst1(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);
> }
> 0000000000004608 <arm_smmu_cmdq_build_cmd_tst1>:
>      4608:       a9007c1f        stp     xzr, xzr, [x0]
>      460c:       39400022        ldrb    w2, [x1]
>      4610:       f9400001        ldr     x1, [x0]
>      4614:       aa020021        orr     x1, x1, x2
>      4618:       f9000001        str     x1, [x0]
>      461c:       d65f03c0        ret
> 
> Case 2):
> void arm_smmu_cmdq_build_cmd_tst2(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
> {
>          int i;
> 
>          cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);
>          for (i = 1; i < CMDQ_ENT_DWORDS; i++)
>                  cmd[i] = 0;
> }
> 0000000000004620 <arm_smmu_cmdq_build_cmd_tst2>:
>      4620:       39400021        ldrb    w1, [x1]
>      4624:       a9007c01        stp     x1, xzr, [x0]
>      4628:       d65f03c0        ret
>      462c:       d503201f        nop
> 
> Case 3):
> void arm_smmu_cmdq_build_cmd_tst3(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);
> }
> 0000000000004630 <arm_smmu_cmdq_build_cmd_tst3>:
>      4630:       a9007c1f        stp     xzr, xzr, [x0]
>      4634:       39400021        ldrb    w1, [x1]
>      4638:       f9000001        str     x1, [x0]
>      463c:       d65f03c0        ret
>