linux-kernel - Re: [PATCH 3/4] iommu/amd: Do not sync on page size changes

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Message-ID: <F5D25BE7-FA34-4017-AA22-DDAB24F634BC@vmware.com>
Date:   Tue, 1 Jun 2021 16:39:54 +0000
From:   Nadav Amit <namit@...are.com>
To:     Robin Murphy <robin.murphy@....com>
CC:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Jiajun Cao <caojiajun@...are.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] iommu/amd: Do not sync on page size changes



> On Jun 1, 2021, at 8:59 AM, Robin Murphy <robin.murphy@....com> wrote:
> 
> On 2021-05-02 07:59, Nadav Amit wrote:
>> From: Nadav Amit <namit@...are.com>
>> Some IOMMU architectures perform invalidations regardless of the page
>> size. In such architectures there is no need to sync when the page size
>> changes or to regard pgsize when making interim flush in
>> iommu_iotlb_gather_add_page().
>> Add a "ignore_gather_pgsize" property for each IOMMU-ops to decide
>> whether gather's pgsize is tracked and triggers a flush.
> 
> I've objected before[1], and I'll readily object again ;)
> 
> I still think it's very silly to add a bunch of indirection all over the place to make a helper function not do the main thing it's intended to help with. If you only need trivial address gathering then it's far simpler to just implement trivial address gathering. I suppose if you really want to you could factor out another helper to share the 5 lines of code which ended up in mtk-iommu (see commit f21ae3b10084).

Thanks, Robin.

I read your comments but I cannot fully understand the alternative that you propose, although I do understand your objection to the indirection “ignore_gather_pgsize”. Would it be ok if “ignore_gather_pgsize" was provided as an argument for iommu_iotlb_gather_add_page()?

In general, I can live without this patch. It probably would have negligent impact on performance anyhow.

Regards,
Nadav