linux-kernel - Re: [PATCH] iommu/dma-iommu: properly respect configured address space size

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Message-ID: <68e7a18b-739e-b73e-eacf-3cb6c1bd279a@arm.com>
Date:   Tue, 8 Nov 2016 11:37:23 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Marek Szyprowski <m.szyprowski@...sung.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Cc:     Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] iommu/dma-iommu: properly respect configured address
 space size

Hi Marek,

On 07/11/16 13:06, Marek Szyprowski wrote:
> When one called iommu_dma_init_domain() with size smaller than device's
> DMA mask, the alloc_iova() will not respect it and always assume that all
> IOVA addresses will be allocated from the the (base ... dev->dma_mask) range.

Is that actually a problem for anything?

> This patch fixes this issue by taking the configured address space size
> parameter into account (if it is smaller than the device's dma_mask).

TBH I've been pondering ripping the size stuff out of dma-iommu, as it
all stems from me originally failing to understand what dma_32bit_pfn is
actually for. The truth is that iova_domains just don't have a size or
upper limit; however if devices with both large and small DMA masks
share a domain, then the top-down nature of the allocator means that
allocating for the less-capable devices would involve walking through
every out-of-range entry in the tree every time. Having cached32_node
based on dma_32bit_pfn just provides an optimised starting point for
searching within the smaller mask.

Would it hurt any of your use-cases to relax/rework the reinitialisation
checks in iommu_dma_init_domain()? Alternatively if we really do have a
case for wanting a hard upper limit, it might make more sense to add an
end_pfn to the iova_domain and handle it in the allocator itself.

Robin.

> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> ---
>  drivers/iommu/dma-iommu.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index c5ab8667e6f2..8b4b72654359 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -212,11 +212,13 @@ static struct iova *__alloc_iova(struct iommu_domain *domain, size_t size,
>  
>  	if (domain->geometry.force_aperture)
>  		dma_limit = min(dma_limit, domain->geometry.aperture_end);
> +
> +	dma_limit = min(dma_limit >> shift, (dma_addr_t)iovad->dma_32bit_pfn);
>  	/*
>  	 * Enforce size-alignment to be safe - there could perhaps be an
>  	 * attribute to control this per-device, or at least per-domain...
>  	 */
> -	return alloc_iova(iovad, length, dma_limit >> shift, true);
> +	return alloc_iova(iovad, length, dma_limit, true);
>  }
>  
>  /* The IOVA allocator knows what we mapped, so just unmap whatever that was */
>