Re: [PATCH v5 2/3] iommu/dma: Reserve IOVA for PCIe inaccessible DMA address

[Robin Murphy]

Hi Lorenzo,

On 02/05/2019 12:01, Lorenzo Pieralisi wrote:
> On Wed, May 01, 2019 at 11:06:25PM +0530, Srinath Mannam wrote:
> > dma_ranges field of PCI host bridge structure has resource entries in
> > sorted order of address range given through dma-ranges DT property. This
> > list is the accessible DMA address range. So that this resource list will
> > be processed and reserve IOVA address to the inaccessible address holes in
> > the list.
> >
> > This method is similar to PCI IO resources address ranges reserving in
> > IOMMU for each EP connected to host bridge.
> >
> > Signed-off-by: Srinath Mannam <srinath.mannam@xxxxxxxxxxxx>
> > Based-on-patch-by: Oza Pawandeep <oza.oza@xxxxxxxxxxxx>
> > Reviewed-by: Oza Pawandeep <poza@xxxxxxxxxxxxxx>
> > Acked-by: Robin Murphy <robin.murphy@xxxxxxx>
> > ---
> >   drivers/iommu/dma-iommu.c | 19 +++++++++++++++++++
> >   1 file changed, 19 insertions(+)
> >
> > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > index 77aabe6..da94844 100644
> > --- a/drivers/iommu/dma-iommu.c
> > +++ b/drivers/iommu/dma-iommu.c
> > @@ -212,6 +212,7 @@ static void iova_reserve_pci_windows(struct pci_dev *dev,
> >   	struct pci_host_bridge *bridge = pci_find_host_bridge(dev->bus);
> >   	struct resource_entry *window;
> >   	unsigned long lo, hi;
> > +	phys_addr_t start = 0, end;
> >   
> >   	resource_list_for_each_entry(window, &bridge->windows) {
> >   		if (resource_type(window->res) != IORESOURCE_MEM)
> > @@ -221,6 +222,24 @@ static void iova_reserve_pci_windows(struct pci_dev *dev,
> >   		hi = iova_pfn(iovad, window->res->end - window->offset);
> >   		reserve_iova(iovad, lo, hi);
> >   	}
> > +
> > +	/* Get reserved DMA windows from host bridge */
> > +	resource_list_for_each_entry(window, &bridge->dma_ranges) {
>
> If this list is not sorted it seems to me the logic in this loop is
> broken and you can't rely on callers to sort it because it is not a
> written requirement and it is not enforced (you know because you
> wrote the code but any other developer is not supposed to guess
> it).
>
> Can't we rewrite this loop so that it does not rely on list
> entries order ?

The original idea was that callers should be required to provide a 
sorted list, since it keeps things nice and simple...

> I won't merge this series unless you sort it, no pun intended.
>
> Lorenzo
>
> > +		end = window->res->start - window->offset;

...so would you consider it sufficient to add

		if (end < start)
			dev_err(...);

here, plus commenting the definition of pci_host_bridge::dma_ranges that 
it must be sorted in ascending order?

[ I guess it might even make sense to factor out the parsing and list 
construction from patch #3 into an of_pci core helper from the 
beginning, so that there's even less chance of another driver 
reimplementing it incorrectly in future. ]

Failing that, although I do prefer the "simple by construction" 
approach, I'd have no objection to just sticking a list_sort() call in 
here instead, if you'd rather it be entirely bulletproof.

Robin.

> > +resv_iova:
> > +		if (end - start) {
> > +			lo = iova_pfn(iovad, start);
> > +			hi = iova_pfn(iovad, end);
> > +			reserve_iova(iovad, lo, hi);
> > +		}
> > +		start = window->res->end - window->offset + 1;
> > +		/* If window is last entry */
> > +		if (window->node.next == &bridge->dma_ranges &&
> > +		    end != ~(dma_addr_t)0) {
> > +			end = ~(dma_addr_t)0;
> > +			goto resv_iova;
> > +		}
> > +	}
> >   }
> >   
> >   static int iova_reserve_iommu_regions(struct device *dev,
> > --
> > 2.7.4