Re: [PATCH] iommu/vt-d: gracefully handle DMAR units with no supported address widths

David Woodhouse
2021-01-20T15:58:57Z

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On Wed, 2021-01-20 at 13:06 +0100, Greg KH wrote:
> On Wed, Jan 20, 2021 at 09:42:43AM +0000, David Woodhouse wrote:
> > On Thu, 2020-09-24 at 15:08 +0100, David Woodhouse wrote:
> > > From: David Woodhouse <dwmw@amazon.co.uk>
> > >
> > > Instead of bailing out completely, such a unit can still be used for
> > > interrupt remapping.
> > >
> > > Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> >
> > Could we have this for stable too please, along with the trivial
> > subsequent fixup. They are:
> >
> > c40aaaac1018 ("iommu/vt-d: Gracefully handle DMAR units with no supported address widths")
> > 9def3b1a07c4 ("iommu/vt-d: Don't dereference iommu_device if IOMMU_API is not built")
> >
> > They apply fairly straightforwardly when backported; let me know if you
> > want us to send patches.
>
> What stable kernel(s) do you want this in? The above patches are
> already in 5.10.

It's a fairly simple bug fix, to still use a given IOMMU for interrupt
remapping even if it can't be used for DMA mapping.

Those features are somewhat orthogonal, and it was wrong for the kernel
to bail out on the IOMMU hardware completely.

The interrupt remapping support is what's required for Intel boxes (or
VMs) to run with more than 255 CPUs. It should be fairly simple to fix
the same bug at least as far back as 4.14.