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From: John Garry <john.garry@huawei.com>
To: Robin Murphy <robin.murphy@arm.com>, <will@kernel.org>
Cc: <joro@8bytes.org>, <linux-arm-kernel@lists.infradead.org>,
	<iommu@lists.linux-foundation.org>,
	<linux-kernel@vger.kernel.org>, <linuxarm@huawei.com>
Subject: Re: [PATCH] iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue_cmdlist()
Date: Thu, 5 Aug 2021 16:16:02 +0100	[thread overview]
Message-ID: <44c5e07b-e663-5b96-a142-ec25666e2a14@huawei.com> (raw)
In-Reply-To: <577a625a-4fc5-7402-8e4f-4e0e5be93144@arm.com>

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

  reply	other threads:[~2021-08-05 15:16 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-21 16:36 [PATCH] iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue_cmdlist() John Garry
2021-08-05 10:22 ` John Garry
2021-08-05 11:21   ` Will Deacon
2021-08-05 11:24 ` Robin Murphy
2021-08-05 12:18   ` Robin Murphy
2021-08-05 13:40   ` John Garry
2021-08-05 14:41     ` Robin Murphy
2021-08-05 15:16       ` John Garry [this message]
2021-08-05 17:14         ` Robin Murphy

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