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

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Message-ID: <578d6aa5-4239-f5d7-2e9f-686b18e52bba@arm.com>
Date:   Tue, 30 Mar 2021 17:28:19 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, 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 14:58, Will Deacon wrote:
> On Tue, Mar 30, 2021 at 02:19:38PM +0100, Robin Murphy wrote:
>> 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.
> 
> Like I say, the cleanup is great. I'm just wondering whether there's a
> better way to express the complicated logic to decide whether or not to use
> the flush queue than what we end up with:
> 
> 	if (!cookie->fq_domain && (!dev || !dev_is_untrusted(dev)) &&
> 	    domain->ops->flush_iotlb_all && !iommu_get_dma_strict())
> 
> which is mixing up globals, device properties and domain properties. The
> result is that the driver code ends up just using the global to determine
> whether or not to pass IO_PGTABLE_QUIRK_NON_STRICT to the page-table code,
> which is a departure from the current way of doing things.

But previously, SMMU only ever saw the global policy piped through the 
domain attribute by iommu_group_alloc_default_domain(), so there's no 
functional change there.

Obviously some of the above checks could be factored out into some kind 
of iommu_use_flush_queue() helper that IOMMU drivers can also call if 
they need to keep in sync. Or maybe we just allow iommu-dma to set 
IO_PGTABLE_QUIRK_NON_STRICT directly via iommu_set_pgtable_quirks() if 
we're treating that as a generic thing now.

>>> 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 :/
> 
> Ah, sorry, I didn't spot that! I was just trying to illustrate that this
> is per-device.

Sure, I understand - and I'm just trying to bang home that despite 
appearances it's never actually been treated as such for SMMU, so 
anything that's wrong after this change was already wrong before.

Robin.