Re: [PATCH] iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue_cmdlist()

On 05/08/2021 15:41, Robin Murphy wrote:

quoted

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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