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

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Date:   Thu, 2 May 2019 12:27:02 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Srinath Mannam <srinath.mannam@...adcom.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Eric Auger <eric.auger@...hat.com>,
        Joerg Roedel <joro@...tes.org>, poza@...eaurora.org,
        Ray Jui <rjui@...adcom.com>,
        bcm-kernel-feedback-list@...adcom.com, linux-pci@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/3] iommu/dma: Reserve IOVA for PCIe inaccessible DMA
 address

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@...adcom.com>
>> Based-on-patch-by: Oza Pawandeep <oza.oza@...adcom.com>
>> Reviewed-by: Oza Pawandeep <poza@...eaurora.org>
>> Acked-by: Robin Murphy <robin.murphy@....com>
>> ---
>>   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
>>