linux-kernel - Re: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL

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Date:   Tue, 2 Nov 2021 07:43:56 +0100
From:   Christoph Hellwig <hch@....de>
To:     Walter Wu <walter-zh.wu@...iatek.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>, Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Linux IOMMU <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        wsd_upstream <wsd_upstream@...iatek.com>,
        linux-mediatek@...ts.infradead.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING

On Tue, Nov 02, 2021 at 11:21:16AM +0800, Walter Wu wrote:
> Our platform is arch64. We need a dynamic allocated buffer from CMA is
> not to read by CPU peculative execution, so we need to remove its
> kernel mapping.

If your CPU speculates into unused kernel direct mappings your have
a worse problem than this, because all the dma coherent allocations for
non-coherent devices still have a cachable direct mapping.  Will
mentioned he wanted to look into getting rid of that mapping now that
the core dma code has the infrastucture for that, so adding him here.