Re: [PATCH] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd()
From: Leizhen (ThunderTown)
Date: Tue Aug 17 2021 - 22:28:29 EST
On 2021/8/17 21:23, Robin Murphy wrote:
> On 2021-08-17 12:34, Zhen Lei 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. The
>> compiler almost always reads the value of cmd[i] from memory rather than
>> directly using the value cached in the register. This generates many
>> useless instruction operations and affects the performance to some extent.
>
> Which compiler? GCC 4.9 does not make the same codegen decisions that GCC 10 does; Clang is different again. There are also various config options which affect a compiler's inlining/optimisation choices either directly or indirectly.
gcc version 7.3.1 20180425 [linaro-7.3-2018.05 revision d29120a424ecfbc167ef90065c0eeb7f91977701] (Linaro GCC 7.3-2018.05)
In addition, yesterday morning I also purposely compiled a compiler with the latest
GCC source code. The result is the same.
gcc version 11.2.0 (GCC)
>
> If it's something that newer compilers can get right anyway, then micro-optimising just for older ones might warrant a bit more justification.
>
>> To guide the compiler for proper optimization, 'cmd' is defined as a local
>> array variable, marked as register, 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
>> 27602 1348 56 29006 714e
>>
>> After:
>> text data bss dec hex
>> 27402 1348 56 28806 7086
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
>> ---
>> drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 18 +++++++++++++++---
>> 1 file changed, 15 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 d76bbbde558b776..50a9db5bac466c7 100644
>> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>> @@ -233,11 +233,19 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent)
>> return 0;
>> }
>> +#define arm_smmu_cmdq_copy_cmd(dst, src) \
>> + do { \
>> + dst[0] = src[0]; \
>> + dst[1] = src[1]; \
>> + } while (0)
>> +
>> /* 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);
>> + register u64 cmd[CMDQ_ENT_DWORDS];
>> +
>> + cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode);
>> + cmd[1] = 0;
>> switch (ent->opcode) {
>> case CMDQ_OP_TLBI_EL2_ALL:
>> @@ -309,6 +317,7 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
>> case PRI_RESP_SUCC:
>> break;
>> default:
>> + arm_smmu_cmdq_copy_cmd(out_cmd, cmd);
>
> Why bother writing back a partial command when we're telling the caller it's invalid anyway?
Some callers do not check the return value of arm_smmu_cmdq_build_cmd().
In particular, the arm_smmu_cmdq_batch_add has no judgment. Yes, I should
add judgment there.
>
>> return -EINVAL;
>> }
>> cmd[1] |= FIELD_PREP(CMDQ_PRI_1_RESP, ent->pri.resp);
>> @@ -329,9 +338,12 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
>> cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB);
>> break;
>> default:
>> + arm_smmu_cmdq_copy_cmd(out_cmd, cmd);
>
> Ditto.
>
>> return -ENOENT;
>> }
>> + arm_smmu_cmdq_copy_cmd(out_cmd, cmd);
>
> ...and then it would be simpler to open-code the assignment here.
Right, I'll modify it in v2. I also don't like the addition of arm_smmu_cmdq_copy_cmd().
>
> I guess if you're really concerned with avoiding temporary commands being written back to the stack and reloaded, it might be worth experimenting with wrapping them in a struct which can be passed around by value - AAPCS64 allows passing a 16-byte composite type purely in registers.
'out_cmd' is an output parameter. Use 16-byte composite type need to modify
many functions, this may not be good..
>
> Robin.
>
>> +
>> return 0;
>> }
>>
> .
>