linux-kernel - Re: [RFC] dma-mapping: introduce dma_can_skip

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Message-ID: <20240301064632-mutt-send-email-mst@kernel.org>
Date: Fri, 1 Mar 2024 06:50:05 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Robin Murphy <robin.murphy@....com>
Cc: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, linux-kernel@...r.kernel.org,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>, iommu@...ts.linux.dev,
	Zelin Deng <zelin.deng@...ux.alibaba.com>
Subject: Re: [RFC] dma-mapping: introduce dma_can_skip_unmap()

On Fri, Mar 01, 2024 at 11:38:25AM +0000, Robin Murphy wrote:
> Not only is this idea not viable, the entire premise seems flawed - the
> reasons for virtio needing to use the DMA API at all are highly likely to be
> the same reasons for it needing to use the DMA API *properly* anyway.

The idea has nothing to do with virtio per se - we are likely not the
only driver that wastes a lot of memory (hot in cache, too) keeping DMA
addresses around for the sole purpose of calling DMA unmap.  On a bunch
of systems unmap is always a nop and we could save some memory if there
was a way to find out. What is proposed is an API extension allowing
that for anyone - not just virtio.

-- 
MST