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

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi,

On 12/12/19 4:28 AM, 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>

This fix looks reasonable to me.

Acked-by: Lu Baolu <baolu.lu@linux.intel.com>

Best regards,
baolu

> ---
>   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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