Re: [PATCH V2 2/6] iommu: iova: properly handle 0 as a valid IOVA address

[Robin Murphy <robin.murphy@arm.com>]

On 2022-05-11 13:15, Ajay Kumar wrote:
> From: Marek Szyprowski <m.szyprowski@samsung.com>
> 
> Zero is a valid DMA and IOVA address on many architectures, so adjust the
> IOVA management code to properly handle it. A new value IOVA_BAD_ADDR
> (~0UL) is introduced as a generic value for the error case. Adjust all
> callers of the alloc_iova_fast() function for the new return value.

And when does anything actually need this? In fact if you were to stop 
iommu-dma from reserving IOVA 0 - which you don't - it would only show 
how patch #3 is broken.

Also note that it's really nothing to do with architectures either way; 
iommu-dma simply chooses to reserve IOVA 0 for its own convenience, 
mostly because it can. Much the same way that 0 is typically a valid CPU 
VA, but mapping something meaningful there is just asking for a world of 
pain debugging NULL-dereference bugs.

Robin.

> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
> ---
>   drivers/iommu/dma-iommu.c | 16 +++++++++-------
>   drivers/iommu/iova.c      | 13 +++++++++----
>   include/linux/iova.h      |  1 +
>   3 files changed, 19 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 1ca85d37eeab..16218d6a0703 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -605,7 +605,7 @@ static dma_addr_t iommu_dma_alloc_iova(struct iommu_domain *domain,
>   {
>   	struct iommu_dma_cookie *cookie = domain->iova_cookie;
>   	struct iova_domain *iovad = &cookie->iovad;
> -	unsigned long shift, iova_len, iova = 0;
> +	unsigned long shift, iova_len, iova = IOVA_BAD_ADDR;
>   
>   	if (cookie->type == IOMMU_DMA_MSI_COOKIE) {
>   		cookie->msi_iova += size;
> @@ -625,11 +625,13 @@ static dma_addr_t iommu_dma_alloc_iova(struct iommu_domain *domain,
>   		iova = alloc_iova_fast(iovad, iova_len,
>   				       DMA_BIT_MASK(32) >> shift, false);
>   
> -	if (!iova)
> +	if (iova == IOVA_BAD_ADDR)
>   		iova = alloc_iova_fast(iovad, iova_len, dma_limit >> shift,
>   				       true);
>   
> -	return (dma_addr_t)iova << shift;
> +	if (iova != IOVA_BAD_ADDR)
> +		return (dma_addr_t)iova << shift;
> +	return DMA_MAPPING_ERROR;
>   }
>   
>   static void iommu_dma_free_iova(struct iommu_dma_cookie *cookie,
> @@ -688,7 +690,7 @@ static dma_addr_t __iommu_dma_map(struct device *dev, phys_addr_t phys,
>   	size = iova_align(iovad, size + iova_off);
>   
>   	iova = iommu_dma_alloc_iova(domain, size, dma_mask, dev);
> -	if (!iova)
> +	if (iova == DMA_MAPPING_ERROR)
>   		return DMA_MAPPING_ERROR;
>   
>   	if (iommu_map_atomic(domain, iova, phys - iova_off, size, prot)) {
> @@ -799,7 +801,7 @@ static struct page **__iommu_dma_alloc_noncontiguous(struct device *dev,
>   
>   	size = iova_align(iovad, size);
>   	iova = iommu_dma_alloc_iova(domain, size, dev->coherent_dma_mask, dev);
> -	if (!iova)
> +	if (iova == DMA_MAPPING_ERROR)
>   		goto out_free_pages;
>   
>   	if (sg_alloc_table_from_pages(sgt, pages, count, 0, size, GFP_KERNEL))
> @@ -1204,7 +1206,7 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
>   	}
>   
>   	iova = iommu_dma_alloc_iova(domain, iova_len, dma_get_mask(dev), dev);
> -	if (!iova) {
> +	if (iova == DMA_MAPPING_ERROR) {
>   		ret = -ENOMEM;
>   		goto out_restore_sg;
>   	}
> @@ -1516,7 +1518,7 @@ static struct iommu_dma_msi_page *iommu_dma_get_msi_page(struct device *dev,
>   		return NULL;
>   
>   	iova = iommu_dma_alloc_iova(domain, size, dma_get_mask(dev), dev);
> -	if (!iova)
> +	if (iova == DMA_MAPPING_ERROR)
>   		goto out_free_page;
>   
>   	if (iommu_map(domain, iova, msi_addr, size, prot))
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index db77aa675145..ae0fe0a6714e 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -429,6 +429,8 @@ EXPORT_SYMBOL_GPL(free_iova);
>    * This function tries to satisfy an iova allocation from the rcache,
>    * and falls back to regular allocation on failure. If regular allocation
>    * fails too and the flush_rcache flag is set then the rcache will be flushed.
> + * Returns a pfn the allocated iova starts at or IOVA_BAD_ADDR in the case
> + * of a failure.
>   */
>   unsigned long
>   alloc_iova_fast(struct iova_domain *iovad, unsigned long size,
> @@ -447,7 +449,7 @@ alloc_iova_fast(struct iova_domain *iovad, unsigned long size,
>   		size = roundup_pow_of_two(size);
>   
>   	iova_pfn = iova_rcache_get(iovad, size, limit_pfn + 1);
> -	if (iova_pfn)
> +	if (iova_pfn != IOVA_BAD_ADDR)
>   		return iova_pfn;
>   
>   retry:
> @@ -456,7 +458,7 @@ alloc_iova_fast(struct iova_domain *iovad, unsigned long size,
>   		unsigned int cpu;
>   
>   		if (!flush_rcache)
> -			return 0;
> +			return IOVA_BAD_ADDR;
>   
>   		/* Try replenishing IOVAs by flushing rcache. */
>   		flush_rcache = false;
> @@ -831,7 +833,7 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache,
>   				       unsigned long limit_pfn)
>   {
>   	struct iova_cpu_rcache *cpu_rcache;
> -	unsigned long iova_pfn = 0;
> +	unsigned long iova_pfn = IOVA_BAD_ADDR;
>   	bool has_pfn = false;
>   	unsigned long flags;
>   
> @@ -858,6 +860,9 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache,
>   
>   	spin_unlock_irqrestore(&cpu_rcache->lock, flags);
>   
> +	if (!iova_pfn)
> +		return IOVA_BAD_ADDR;
> +
>   	return iova_pfn;
>   }
>   
> @@ -873,7 +878,7 @@ static unsigned long iova_rcache_get(struct iova_domain *iovad,
>   	unsigned int log_size = order_base_2(size);
>   
>   	if (log_size >= IOVA_RANGE_CACHE_MAX_SIZE || !iovad->rcaches)
> -		return 0;
> +		return IOVA_BAD_ADDR;
>   
>   	return __iova_rcache_get(&iovad->rcaches[log_size], limit_pfn - size);
>   }
> diff --git a/include/linux/iova.h b/include/linux/iova.h
> index 320a70e40233..46b5b10c532b 100644
> --- a/include/linux/iova.h
> +++ b/include/linux/iova.h
> @@ -21,6 +21,7 @@ struct iova {
>   	unsigned long	pfn_lo; /* Lowest allocated pfn */
>   };
>   
> +#define IOVA_BAD_ADDR	(~0UL)
>   
>   struct iova_rcache;
>   
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