Received-SPF: pass (google.com: domain of linux-kernel+bounces-84994-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Date: Wed, 28 Feb 2024 12:05:23 +0000
From: Will Deacon <will@kernel.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Christoph Hellwig <hch@infradead.org>, 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>
Subject: Re: [PATCH v4 5/5] iommu/dma: Force swiotlb_max_mapping_size on an
 untrusted device
Message-ID: <20240228120523.GA15054@willie-the-truck>
References: <20240221113504.7161-1-will@kernel.org>
 <20240221113504.7161-6-will@kernel.org>
 <Zd4CcL3Nnvybw2xF@infradead.org>
 <57c928e6-14a4-4724-8c07-e985a2bce522@arm.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <57c928e6-14a4-4724-8c07-e985a2bce522@arm.com>
User-Agent: Mutt/1.10.1 (2018-07-13)

On Tue, Feb 27, 2024 at 03:53:05PM +0000, Robin Murphy wrote:
> 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.

Makes sense. Since this is all internal to the IOMMU DMA code, I can just
drop the first part of the check.

I'll get a v5 out shortly.

Will