linux-kernel - Re: [PATCH v15 06/12] swiotlb: Use is_swiotlb_force

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Message-ID: <20210706044848.GA13640@lst.de>
Date:   Tue, 6 Jul 2021 06:48:48 +0200
From:   Christoph Hellwig <hch@....de>
To:     Will Deacon <will@...nel.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Claire Chang <tientzu@...omium.org>,
        Rob Herring <robh+dt@...nel.org>, mpe@...erman.id.au,
        Joerg Roedel <joro@...tes.org>,
        Frank Rowand <frowand.list@...il.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        benh@...nel.crashing.org, paulus@...ba.org,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        grant.likely@....com, xypron.glpk@....de,
        Thierry Reding <treding@...dia.com>, mingo@...nel.org,
        bauerman@...ux.ibm.com, peterz@...radead.org,
        Greg KH <gregkh@...uxfoundation.org>,
        Saravana Kannan <saravanak@...gle.com>,
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        heikki.krogerus@...ux.intel.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        linux-devicetree <devicetree@...r.kernel.org>,
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        Bjorn Helgaas <bhelgaas@...gle.com>, chris@...is-wilson.co.uk,
        Daniel Vetter <daniel@...ll.ch>, airlied@...ux.ie,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        jani.nikula@...ux.intel.com, Jianxiong Gao <jxgao@...gle.com>,
        joonas.lahtinen@...ux.intel.com, linux-pci@...r.kernel.org,
        maarten.lankhorst@...ux.intel.com, matthew.auld@...el.com,
        rodrigo.vivi@...el.com, thomas.hellstrom@...ux.intel.com,
        Tom Lendacky <thomas.lendacky@....com>,
        Qian Cai <quic_qiancai@...cinc.com>
Subject: Re: [PATCH v15 06/12] swiotlb: Use is_swiotlb_force_bounce for
 swiotlb data bouncing

On Mon, Jul 05, 2021 at 08:03:52PM +0100, Will Deacon wrote:
> So at this point, the AMD IOMMU driver does:
> 
> 	swiotlb        = (iommu_default_passthrough() || sme_me_mask) ? 1 : 0;
> 
> where 'swiotlb' is a global variable indicating whether or not swiotlb
> is in use. It's picked up a bit later on by pci_swiotlb_late_init(), which
> will call swiotlb_exit() if 'swiotlb' is false.
> 
> Now, that used to work fine, because swiotlb_exit() clears
> 'io_tlb_default_mem' to NULL, but now with the restricted DMA changes, I
> think that all the devices which have successfully probed beforehand will
> have stale pointers to the freed structure in their 'dev->dma_io_tlb_mem'
> field.

Yeah.  I don't think we can do that anymore, and I also think it is
a bad idea to start with.