[PATCH] iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue_cmdlist()
John Garry
john.garry at huawei.com
Thu Aug 5 08:16:02 PDT 2021
On 05/08/2021 15:41, Robin Murphy wrote:
>> I suppose they could be combined into a smaller sub-struct and loaded
>> in a single operation, but it looks messy, and prob without much gain.
>
> Indeed I wouldn't say that saving memory is the primary concern here,
> and any more convoluted code is hardly going to help performance. Plus
> it still wouldn't help the other cases where we're just copying the size
> into a fake queue to do some prod arithmetic - I hadn't fully clocked
> what was going on there when I skimmed through things earlier.
>
> Disregarding the bogus layout change, though, do you reckon the rest of
> my idea makes sense?
I tried the similar change to avoid zero-init the padding in
arm_smmu_cmdq_write_entries() and the
_arm_smmu_cmdq_poll_set_valid_map(), but the disassembly was the same.
So the compiler must have got smart there.
But for the original change in this patch, it did make a difference.
It's nice to remove what was a memcpy:
1770: a9077eff stp xzr, xzr, [x23, #112]
}, head = llq;
1774: 94000000 bl 0 <memcpy>
And performance was very fractionally better.
As for pre-evaluating "nents", I'm not sure how much that can help, but
I am not too optimistic. I can try some testing when I get a chance.
Having said that, I would need to check the disassembly also.
Thanks,
John
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