Re: [PATCH] iommu/vt-d: Set ISA bridge reserved region as relaxable

[Jerry Snitselaar <jsnitsel@redhat.com>]

On Wed Dec 11 19, Alex Williamson wrote:
>Commit d850c2ee5fe2 ("iommu/vt-d: Expose ISA direct mapping region via
>iommu_get_resv_regions") created a direct-mapped reserved memory region
>in order to replace the static identity mapping of the ISA address
>space, where the latter was then removed in commit df4f3c603aeb
>("iommu/vt-d: Remove static identity map code").  According to the
>history of this code and the Kconfig option surrounding it, this direct
>mapping exists for the benefit of legacy ISA drivers that are not
>compatible with the DMA API.
>
>In conjuntion with commit 9b77e5c79840 ("vfio/type1: check dma map
>request is within a valid iova range") this change introduced a
>regression where the vfio IOMMU backend enforces reserved memory regions
>per IOMMU group, preventing userspace from creating IOMMU mappings
>conflicting with prescribed reserved regions.  A necessary prerequisite
>for the vfio change was the introduction of "relaxable" direct mappings
>introduced by commit adfd37382090 ("iommu: Introduce
>IOMMU_RESV_DIRECT_RELAXABLE reserved memory regions").  These relaxable
>direct mappings provide the same identity mapping support in the default
>domain, but also indicate that the reservation is software imposed and
>may be relaxed under some conditions, such as device assignment.
>
>Convert the ISA bridge direct-mapped reserved region to relaxable to
>reflect that the restriction is self imposed and need not be enforced
>by drivers such as vfio.
>
>Fixes: d850c2ee5fe2 ("iommu/vt-d: Expose ISA direct mapping region via iommu_get_resv_regions")
>Cc: stable@vger.kernel.org # v5.3+
>Link: https://lore.kernel.org/linux-iommu/20191211082304.2d4fab45@x1.home
>Reported-by: cprt <cprt@protonmail.com>
>Tested-by: cprt <cprt@protonmail.com>
>Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Tested-by: Jerry Snitselaar <jsnitsel@redhat.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>

>---
> drivers/iommu/intel-iommu.c |    2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
>index 0c8d81f56a30..6eb0dd7489a1 100644
>--- a/drivers/iommu/intel-iommu.c
>+++ b/drivers/iommu/intel-iommu.c
>@@ -5737,7 +5737,7 @@ static void intel_iommu_get_resv_regions(struct device *device,
>
> 		if ((pdev->class >> 8) == PCI_CLASS_BRIDGE_ISA) {
> 			reg = iommu_alloc_resv_region(0, 1UL << 24, 0,
>-						      IOMMU_RESV_DIRECT);
>+						   IOMMU_RESV_DIRECT_RELAXABLE);
> 			if (reg)
> 				list_add_tail(&reg->list, head);
> 		}
>
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