linux-kernel - Re: dma_map_resource() has a bad performance in pcie peer to peer transactions when iommu enabled in Linux

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Message-ID: <ZRpjdPC4eSCA/jOk@infradead.org>
Date:   Sun, 1 Oct 2023 23:30:12 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Kelly Devilliv <kelly.devilliv@...look.com>
Cc:     Christian König <christian.koenig@....com>,
        Robin Murphy <robin.murphy@....com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: dma_map_resource() has a bad performance in pcie peer to peer
 transactions when iommu enabled in Linux

FYI, dma_map_resources must never be used for PCIe P2P mappings, you
absolutely must use dma_map_sg as only that handles all the corner cases
correctly.

And we need to figure out a way to put a check into dma_map_resources
so that people stop abusing it.