netdev - Re: [PATCH 14/17] iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH

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Message-ID: <20210310091501.GC5928@lst.de>
Date:   Wed, 10 Mar 2021 10:15:01 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>, Li Yang <leoyang.li@....com>,
        freedreno@...ts.freedesktop.org, kvm@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        iommu@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-arm-msm@...r.kernel.org,
        David Woodhouse <dwmw2@...radead.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 14/17] iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE

On Thu, Mar 04, 2021 at 03:25:27PM +0000, Robin Murphy wrote:
> On 2021-03-01 08:42, Christoph Hellwig wrote:
>> Use explicit methods for setting and querying the information instead.
>
> Now that everyone's using iommu-dma, is there any point in bouncing this 
> through the drivers at all? Seems like it would make more sense for the x86 
> drivers to reflect their private options back to iommu_dma_strict (and 
> allow Intel's caching mode to override it as well), then have 
> iommu_dma_init_domain just test !iommu_dma_strict && 
> domain->ops->flush_iotlb_all.

Hmm.  I looked at this, and kill off ->dma_enable_flush_queue for
the ARM drivers and just looking at iommu_dma_strict seems like a
very clear win.

OTOH x86 is a little more complicated.  AMD and intel defaul to lazy
mode, so we'd have to change the global iommu_dma_strict if they are
initialized.  Also Intel has not only a "static" option to disable
lazy mode, but also a "dynamic" one where it iterates structure.  So
I think on the get side we're stuck with the method, but it still
simplifies the whole thing.