Re: [PATCH v1 2/6] arm/smmu: Add auxiliary domain support for arm-smmuv2

From: Will Deacon <will@kernel.org>
To: Jordan Crouse <jcrouse@codeaurora.org>
Cc: linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org,
	iommu@lists.linux-foundation.org, robin.murphy@arm.com,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v1 2/6] arm/smmu: Add auxiliary domain support for arm-smmuv2
Date: Wed, 18 Mar 2020 22:48:40 +0000	[thread overview]
Message-ID: <20200318224840.GA10796@willie-the-truck> (raw)
In-Reply-To: <1580249770-1088-3-git-send-email-jcrouse@codeaurora.org>

On Tue, Jan 28, 2020 at 03:16:06PM -0700, Jordan Crouse wrote:
> Support auxiliary domains for arm-smmu-v2 to initialize and support
> multiple pagetables for a single SMMU context bank. Since the smmu-v2
> hardware doesn't have any built in support for switching the pagetable
> base it is left as an exercise to the caller to actually use the pagetable.
> 
> Aux domains are supported if split pagetable (TTBR1) support has been
> enabled on the master domain.  Each auxiliary domain will reuse the
> configuration of the master domain. By default the a domain with TTBR1
> support will have the TTBR0 region disabled so the first attached aux
> domain will enable the TTBR0 region in the hardware and conversely the
> last domain to be detached will disable TTBR0 translations.  All subsequent
> auxiliary domains create a pagetable but not touch the hardware.
> 
> The leaf driver will be able to query the physical address of the
> pagetable with the DOMAIN_ATTR_PTBASE attribute so that it can use the
> address with whatever means it has to switch the pagetable base.
> 
> Following is a pseudo code example of how a domain can be created
> 
>  /* Check to see if aux domains are supported */
>  if (iommu_dev_has_feature(dev, IOMMU_DEV_FEAT_AUX)) {
> 	 iommu = iommu_domain_alloc(...);
> 
> 	 if (iommu_aux_attach_device(domain, dev))
> 		 return FAIL;
> 
> 	/* Save the base address of the pagetable for use by the driver
> 	iommu_domain_get_attr(domain, DOMAIN_ATTR_PTBASE, &ptbase);
>  }

I'm not really understanding what the pagetable base gets used for here and,
to be honest with you, the whole thing feels like a huge layering violation
with the way things are structured today. Why doesn't the caller just
interface with io-pgtable directly?

Finally, if we need to support context-switching TTBR0 for a live domain
then that code really needs to live inside the SMMU driver because the
ASID and TLB management necessary to do that safely doesn't belong anywhere
else.

Will
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