Re: [PATCH v2 0/3] arm-smmu: select suitable IOVA

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, May 20, 2025 at 03:42:24PM -0700, Shyam Saini wrote:
> Hi Jason,
> 
> apologies for the delayed response.
> 
> > On Wed, Apr 16, 2025 at 11:04:27AM -0700, Jacob Pan wrote:
> > 
> > > Per last discussion "SMMU driver have a list of potential addresses and
> > > select the first one that does not intersect with the non-working IOVA
> > > ranges.". If we don't know what the "non-working IOVA" is, how do we
> > > know it does not intersect the "potential addresses"?
> > 
> > I had understood from previous discussions that this platform is
> > properly creating IOMMU_RESV_RESERVED regions for the IOVA that
> > doesn't work. Otherwise everything is broken..
> > 
> > Presumably that happens through iommu_dma_get_resv_regions() calling
> > of_iommu_get_resv_regions() on a DT platform. There is a schema
> > describing how to do this, so platform firmware should be able to do it..
> > 
> > So the fix seems trivial enough to me:
> > 
> > diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > index b4c21aaed1266a..ebba18579151bc 100644
> > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > @@ -3562,17 +3562,29 @@ static int arm_smmu_of_xlate(struct device *dev,
> >  static void arm_smmu_get_resv_regions(struct device *dev,
> >  				      struct list_head *head)
> >  {
> > -	struct iommu_resv_region *region;
> > -	int prot = IOMMU_WRITE | IOMMU_NOEXEC | IOMMU_MMIO;
> > -
> > -	region = iommu_alloc_resv_region(MSI_IOVA_BASE, MSI_IOVA_LENGTH,
> > -					 prot, IOMMU_RESV_SW_MSI, GFP_KERNEL);
> > -	if (!region)
> > -		return;
> > -
> > -	list_add_tail(&region->list, head);
> > +	static const u64 msi_bases[] = { MSI_IOVA_BASE, 0x12340000 };
> >  
> >  	iommu_dma_get_resv_regions(dev, head);
> 
> my understand is, this hook is not called for all the devices, eg: pcie dts node
> doesn't use [1] "iommus" property instead it uses "iommu-map" property
> as a consequence, [1] while loop exits prematurely and iommu_dma_get_resv_regions()
> is not called, so there is no IOVA reservation for the pcie device. 

I can't really understand this sentance.

The above is the only place that creates a IOMMU_RESV_SW_MSI so it is
definately called and used, right? If not where does your
IOMMU_RESV_SW_MSI come from?

This function is also the only thing that computes the reserved ranges
that iommu_get_resv_regions() returns.

As above, I've asked a few times now if your resv_regions() is
correct, meaning there is a reserved range covering the address space
that doesn't have working translation. That means
iommu_get_resv_regions() returns such a range.

If you don't have that then you have a bigger platform problem, IMHO,
as vfio/iommufd only respect reserved ranges.

Otherwise, what is the issue you see, exactly? Did you even try it?

Jason