linux-kernel - Re: [PATCH] iommu/dma: Zero pages manually in a length of scatterlist

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Message-ID: <20181109074510.GA27623@infradead.org>
Date:   Thu, 8 Nov 2018 23:45:10 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Nicolin Chen <nicoleotsuka@...il.com>, joro@...tes.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/dma: Zero pages manually in a length of scatterlist

On Tue, Nov 06, 2018 at 02:39:26PM +0000, Robin Murphy wrote:
> ...and I have some significant objections to that simplification which I
> plan to respond with ;)
> 
> (namely that it defaults the whole higher-order page allocation business
> which will have varying degrees of performance impact on certain cases)

Well, please place your objection there.  The behavior does match what
every other iommu-based dma ops implementation ouside of arm/arm64 does,
so there is some precedent for it to say the least.  But if the only
current users objects I'll surely find a way to accomodate it, but a
good rationale including numbers would be useful to document it.