netdev - Re: [PATCH 16/18] iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH

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Message-ID: <a6952aa7-4d7e-54f0-339e-e15f88596dcc@arm.com>
Date:   Tue, 30 Mar 2021 14:19:38 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Will Deacon <will@...nel.org>, Christoph Hellwig <hch@....de>
Cc:     Joerg Roedel <joro@...tes.org>, Li Yang <leoyang.li@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH 16/18] iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE

On 2021-03-30 14:11, Will Deacon wrote:
> On Tue, Mar 16, 2021 at 04:38:22PM +0100, Christoph Hellwig wrote:
>> From: Robin Murphy <robin.murphy@....com>
>>
>> Instead make the global iommu_dma_strict paramete in iommu.c canonical by
>> exporting helpers to get and set it and use those directly in the drivers.
>>
>> This make sure that the iommu.strict parameter also works for the AMD and
>> Intel IOMMU drivers on x86.  As those default to lazy flushing a new
>> IOMMU_CMD_LINE_STRICT is used to turn the value into a tristate to
>> represent the default if not overriden by an explicit parameter.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@....com>.
>> [ported on top of the other iommu_attr changes and added a few small
>>   missing bits]
>> Signed-off-by: Christoph Hellwig <hch@....de>
>> ---
>>   drivers/iommu/amd/iommu.c                   | 23 +-------
>>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 50 +---------------
>>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h |  1 -
>>   drivers/iommu/arm/arm-smmu/arm-smmu.c       | 27 +--------
>>   drivers/iommu/dma-iommu.c                   |  9 +--
>>   drivers/iommu/intel/iommu.c                 | 64 ++++-----------------
>>   drivers/iommu/iommu.c                       | 27 ++++++---
>>   include/linux/iommu.h                       |  4 +-
>>   8 files changed, 40 insertions(+), 165 deletions(-)
> 
> I really like this cleanup, but I can't help wonder if it's going in the
> wrong direction. With SoCs often having multiple IOMMU instances and a
> distinction between "trusted" and "untrusted" devices, then having the
> flush-queue enabled on a per-IOMMU or per-domain basis doesn't sound
> unreasonable to me, but this change makes it a global property.

The intent here was just to streamline the existing behaviour of 
stuffing a global property into a domain attribute then pulling it out 
again in the illusion that it was in any way per-domain. We're still 
checking dev_is_untrusted() before making an actual decision, and it's 
not like we can't add more factors at that point if we want to.

> For example, see the recent patch from Lu Baolu:
> 
> https://lore.kernel.org/r/20210225061454.2864009-1-baolu.lu@linux.intel.com

Erm, this patch is based on that one, it's right there in the context :/

Thanks,
Robin.