linux-kernel - Re: [PATCH v2 3/4] iommu/dma-iommu: Use the dev->coherent_dma

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Date:   Mon, 6 May 2019 18:56:13 +0100
From:   Tom Murphy <tmurphy@...sta.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        iommu@...ts.linux-foundation.org, Heiko Stuebner <heiko@...ech.de>,
        Will Deacon <will.deacon@....com>,
        David Brown <david.brown@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-s390@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        linux-rockchip@...ts.infradead.org, Kukjin Kim <kgene@...nel.org>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Andy Gross <andy.gross@...aro.org>,
        linux-tegra@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Tom Murphy <murphyt7@....ie>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v2 3/4] iommu/dma-iommu: Use the dev->coherent_dma_mask

Just to make this clear, I won't apply Christoph's patch (the one in
this email thread) and instead the only change I will make is to
rename dma_limit to dma_mask.

On Tue, Apr 30, 2019 at 1:05 PM Robin Murphy <robin.murphy@....com> wrote:
>
> On 30/04/2019 12:32, Christoph Hellwig wrote:
> > On Tue, Apr 30, 2019 at 12:27:02PM +0100, Robin Murphy wrote:
> >>> Hmm, I don't think we need the DMA mask for the MSI mapping, this
> >>> should probably always use a 64-bit mask.
> >>
> >> If that were true then we wouldn't need DMA masks for regular mappings
> >> either. If we have to map the MSI doorbell at all, then we certainly have to
> >> place it at an IOVA that the relevant device is actually capable of
> >> addressing.
> >
> > Well, as shown by the patch below we don't even look at the DMA mask
> > for the MSI page - we just allocate from bottom to top.
>
> In the trivial cookie for unmanaged domains, yes, but in that case the
> responsibility is on VFIO to provide a suitable (i.e. sub-32-bit)
> address range for that cookie in the first place. In the managed case,
> allocation uses the streaming mask via iommu_dma_get_msi_page() calling
> __iommu_dma_map(). Admittedly the mask can then get overlooked when
> reusing an existing mapping, which strictly could pose a problem if you
> have multiple devices with incompatible masks in the same group (and
> such that the PCI stuff doesn't already mitigate it), but that's such an
> obscure corner case that I'm reticent to introduce the complication to
> handle it until it's actually proven necessary.
>
> Robin.