[PATCH] iommu: Don't reserve IOVA when address and size are zero

[PATCH] iommu: Don't reserve IOVA when address and size are zero

[Ashish Mhetre]

When the bootloader/firmware doesn't setup the framebuffers, their
address and size are zero in "iommu-addresses" property. If we intend to
use display driver in kernel without framebuffer then it's causing
the display IOMMU mappings to fail as IOVA is reserved with size and
address as zero.
An ideal solution would be firmware removing the "iommu-addresses"
property and corresponding "memory-region" if display is not present.
But the kernel should be able to handle this by checking for size and
address of IOVA and skipping the IOVA reservation if both are 0.

Fixes: a5bf3cfce8cb ("iommu: Implement of_iommu_get_resv_regions()")
Signed-off-by: Ashish Mhetre <amhetre@nvidia.com>
---
 drivers/iommu/of_iommu.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
index 157b286e36bf..150ef65d357a 100644
--- a/drivers/iommu/of_iommu.c
+++ b/drivers/iommu/of_iommu.c
@@ -255,6 +255,10 @@ void of_iommu_get_resv_regions(struct device *dev, struct list_head *list)
 				size_t length;
 
 				maps = of_translate_dma_region(np, maps, &iova, &length);
+				if (iova == 0 && length == 0) {
+					dev_dbg(dev, "Skipping IOVA reservation as address and size are zero\n");
+					continue;
+				}
 				type = iommu_resv_region_get_type(dev, &phys, iova, length);
 
 				region = iommu_alloc_resv_region(iova, length, prot, type,
-- 
2.17.1

Re: [PATCH] iommu: Don't reserve IOVA when address and size are zero

[Robin Murphy]

On 2023-11-23 6:12 am, Ashish Mhetre wrote:
> When the bootloader/firmware doesn't setup the framebuffers, their
> address and size are zero in "iommu-addresses" property. If we intend to
> use display driver in kernel without framebuffer then it's causing
> the display IOMMU mappings to fail as IOVA is reserved with size and
> address as zero.

Can you clarify the problem there? Looking at the code in 
iova_reserve_iommu_regions() I'm guessing it's that "region->start + 
region->length - 1" underflows so reserve_iova() actually ends up 
reserving the entire valid IOVA space?

> An ideal solution would be firmware removing the "iommu-addresses"
> property and corresponding "memory-region" if display is not present.
> But the kernel should be able to handle this by checking for size and
> address of IOVA and skipping the IOVA reservation if both are 0.

Surely it doesn't make sense to reserve a 0-length region at *any* base 
address? The symptom above wouldn't be quite the same if the base was 
nonzero, but corrupting the rbtree with an entry where pfn_hi < pfn_lo 
would definitely not be good either.

> Fixes: a5bf3cfce8cb ("iommu: Implement of_iommu_get_resv_regions()")
> Signed-off-by: Ashish Mhetre <amhetre@nvidia.com>
> ---
>   drivers/iommu/of_iommu.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
> index 157b286e36bf..150ef65d357a 100644
> --- a/drivers/iommu/of_iommu.c
> +++ b/drivers/iommu/of_iommu.c
> @@ -255,6 +255,10 @@ void of_iommu_get_resv_regions(struct device *dev, struct list_head *list)
>   				size_t length;
>   
>   				maps = of_translate_dma_region(np, maps, &iova, &length);
> +				if (iova == 0 && length == 0) {
> +					dev_dbg(dev, "Skipping IOVA reservation as address and size are zero\n");

FWIW I'd be inclined to log a visible warning that firmware is giving us 
nonsense.

Thanks,
Robin.

> +					continue;
> +				}
>   				type = iommu_resv_region_get_type(dev, &phys, iova, length);
>   
>   				region = iommu_alloc_resv_region(iova, length, prot, type,

Re: [PATCH] iommu: Don't reserve IOVA when address and size are zero

[Ashish Mhetre]

On 11/23/2023 4:43 PM, Robin Murphy wrote:
> External email: Use caution opening links or attachments
>
>
> On 2023-11-23 6:12 am, Ashish Mhetre wrote:
>> When the bootloader/firmware doesn't setup the framebuffers, their
>> address and size are zero in "iommu-addresses" property. If we intend to
>> use display driver in kernel without framebuffer then it's causing
>> the display IOMMU mappings to fail as IOVA is reserved with size and
>> address as zero.
>
> Can you clarify the problem there? Looking at the code in
> iova_reserve_iommu_regions() I'm guessing it's that "region->start +
> region->length - 1" underflows so reserve_iova() actually ends up
> reserving the entire valid IOVA space?

Yes, that's the problem which lead to dma_map call failures from
display driver. I don't have the logs handy to pin-point the exact
function which failed as this issue was seen before few months.

>
>> An ideal solution would be firmware removing the "iommu-addresses"
>> property and corresponding "memory-region" if display is not present.
>> But the kernel should be able to handle this by checking for size and
>> address of IOVA and skipping the IOVA reservation if both are 0.
>
> Surely it doesn't make sense to reserve a 0-length region at *any* base
> address? The symptom above wouldn't be quite the same if the base was
> nonzero, but corrupting the rbtree with an entry where pfn_hi < pfn_lo
> would definitely not be good either.
>
Agreed, we should restrict reservation for 0-length region at any base.
I will update the condition in next version.


>> Fixes: a5bf3cfce8cb ("iommu: Implement of_iommu_get_resv_regions()")
>> Signed-off-by: Ashish Mhetre <amhetre@nvidia.com>
>> ---
>>   drivers/iommu/of_iommu.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c
>> index 157b286e36bf..150ef65d357a 100644
>> --- a/drivers/iommu/of_iommu.c
>> +++ b/drivers/iommu/of_iommu.c
>> @@ -255,6 +255,10 @@ void of_iommu_get_resv_regions(struct device 
>> *dev, struct list_head *list)
>>                               size_t length;
>>
>>                               maps = of_translate_dma_region(np, 
>> maps, &iova, &length);
>> +                             if (iova == 0 && length == 0) {
>> +                                     dev_dbg(dev, "Skipping IOVA 
>> reservation as address and size are zero\n");
>
> FWIW I'd be inclined to log a visible warning that firmware is giving us
> nonsense.
>
Okay, I'll replace dev_dbg() with dev_warn() in next version.


> Thanks,
> Robin.
>
>> + continue;
>> +                             }
>>                               type = iommu_resv_region_get_type(dev, 
>> &phys, iova, length);
>>
>>                               region = iommu_alloc_resv_region(iova, 
>> length, prot, type,