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

[Tom Murphy]

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@xxxxxxx> 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.