Re: [PATCH 02/20] iommu/terga-gart: Replace set_platform_dma_ops() with IOMMU_DOMAIN_PLATFORM

From: Jason Gunthorpe <jgg@nvidia.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Andy Gross <agross@kernel.org>,
	Alim Akhtar <alim.akhtar@samsung.com>,
	Bjorn Andersson <andersson@kernel.org>,
	AngeloGioacchino Del Regno 
	<angelogioacchino.delregno@collabora.com>,
	Baolin Wang <baolin.wang@linux.alibaba.com>,
	Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
	Heiko Stuebner <heiko@sntech.de>,
	iommu@lists.linux.dev, Jernej Skrabec <jernej.skrabec@gmail.com>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Joerg Roedel <joro@8bytes.org>,
	Konrad Dybcio <konrad.dybcio@linaro.org>,
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	Kevin Tian <kevin.tian@intel.com>,
	Nicolin Chen <nicolinc@nvidia.com>,
	Steven Price <steven.price@arm.com>
Subject: Re: [PATCH 02/20] iommu/terga-gart: Replace set_platform_dma_ops() with IOMMU_DOMAIN_PLATFORM
Date: Wed, 3 May 2023 08:01:35 -0300	[thread overview]
Message-ID: <ZFI/D6mnLKYpdIqx@nvidia.com> (raw)
In-Reply-To: <1db712d2-9e33-4183-2766-34e32f170507@arm.com>

On Wed, May 03, 2023 at 10:17:29AM +0100, Robin Murphy wrote:
> On 2023-05-01 19:02, Jason Gunthorpe wrote:
> > tegra-gart seems to be kind of wonky since from the start its 'detach_dev'
> > op doesn't actually touch hardware. It is supposed to empty the GART of
> > all translations loaded into it.
> 
> No, detach should never tear down translations - what if other devices are
> still using the domain?

?? All other drivers do this. The core contract is that this sequence:

   dom = iommu_domain_alloc()
   iommu_attach_device(dom, dev)
   iommu_map(dom,...)
   iommu_detach_device(dom, dev)

Will not continue to have the IOVA mapped to the device. We rely on
this for various error paths.

If the HW is multi-device then it is supposed to have groups.

> > Call this weirdness PLATFORM which keeps the basic original
> > ops->detach_dev() semantic alive without needing much special core code
> > support. I'm guessing it really ends up in a BLOCKING configuration, but
> > without any forced cleanup it is unsafe.
> 
> The GART translation aperture is in physical address space, so the truth is
> that all devices have access to it at the same time as having access to the
> rest of physical address space. Attach/detach here really are only
> bookkeeping for which domain currently owns the aperture.

Oh yuk, that is not an UNMANAGED domain either as we now assume empty
UNMANAGED domains are blocking in the core...

> FWIW I wrote up this patch a while ago, not sure if it needs rebasing
> again...

That looks like the same as this patch, just calling the detach dev
behavior IDENTITY. Can do..

Thanks,
Jason