linux-kernel - Re: [PATCH v4 5/5] iommu/dma: Force swiotlb_max_mapping

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Message-ID: <57c928e6-14a4-4724-8c07-e985a2bce522@arm.com>
Date: Tue, 27 Feb 2024 15:53:05 +0000
From: Robin Murphy <robin.murphy@....com>
To: Christoph Hellwig <hch@...radead.org>, Will Deacon <will@...nel.org>
Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com,
 iommu@...ts.linux.dev, Christoph Hellwig <hch@....de>,
 Marek Szyprowski <m.szyprowski@...sung.com>,
 Petr Tesarik <petr.tesarik1@...wei-partners.com>,
 Dexuan Cui <decui@...rosoft.com>, Nicolin Chen <nicolinc@...dia.com>,
 Michael Kelley <mhklinux@...look.com>
Subject: Re: [PATCH v4 5/5] iommu/dma: Force swiotlb_max_mapping_size on an
 untrusted device

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.