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Date: Tue, 27 Feb 2024 15:53:05 +0000
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Subject: Re: [PATCH v4 5/5] iommu/dma: Force swiotlb_max_mapping_size on an
 untrusted device
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To: Christoph Hellwig <hch@infradead.org>, Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org, kernel-team@android.com,
 iommu@lists.linux.dev, Christoph Hellwig <hch@lst.de>,
 Marek Szyprowski <m.szyprowski@samsung.com>,
 Petr Tesarik <petr.tesarik1@huawei-partners.com>,
 Dexuan Cui <decui@microsoft.com>, Nicolin Chen <nicolinc@nvidia.com>,
 Michael Kelley <mhklinux@outlook.com>
References: <20240221113504.7161-1-will@kernel.org>
 <20240221113504.7161-6-will@kernel.org> <Zd4CcL3Nnvybw2xF@infradead.org>
From: Robin Murphy <robin.murphy@arm.com>
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On 27/02/2024 3:40 pm, Christoph Hellwig wrote:
> On Wed, Feb 21, 2024 at 11:35:04AM +0000, Will Deacon wrote:
>> +static size_t iommu_dma_max_mapping_size(struct device *dev)
>> +{
>> +	if (is_swiotlb_active(dev) && dev_is_untrusted(dev))
>> +		return swiotlb_max_mapping_size(dev);
> 
> Curious: do we really need both checks here?  If swiotlb is active
> for a device (for whatever reason), aren't we then always bound
> by the max size?  If not please add a comment explaining it.
> 

Oh, good point - if we have an untrusted device but SWIOTLB isn't 
initialised for whatever reason, then it doesn't matter what 
max_mapping_size returns because iommu_dma_map_page() is going to bail 
out regardless.

Thanks,
Robin.