linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue

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Date:   Thu, 5 Aug 2021 16:16:02 +0100
From:   John Garry <john.garry@...wei.com>
To:     Robin Murphy <robin.murphy@....com>, <will@...nel.org>
CC:     <joro@...tes.org>, <linux-arm-kernel@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>, <linux-kernel@...r.kernel.org>,
        <linuxarm@...wei.com>
Subject: 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:
>> 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