From: Robin Murphy <robin.murphy@arm.com>
To: Christoph Hellwig <hch@lst.de>
Cc: iommu@lists.linux-foundation.org,
Alexey Kardashevskiy <aik@ozlabs.ru>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: generic DMA bypass flag
Date: Tue, 19 Nov 2019 17:41:58 +0000 [thread overview]
Message-ID: <f2335431-8cd4-e1ab-013d-573d163f4067@arm.com> (raw)
In-Reply-To: <20191116062258.GA8913@lst.de>
On 16/11/2019 6:22 am, Christoph Hellwig wrote:
> On Fri, Nov 15, 2019 at 06:12:48PM +0000, Robin Murphy wrote:
>> And is that any different from where you would choose to "just" set a
>> generic bypass flag?
>
> Same spots, as intel-iommu moves from the identify to a dma domain when
> setting a 32-bit mask. But that means once a 32-bit mask is set we can't
> ever go back to the 64-bit one.
Is that a problem though? It's not safe in general to rewrite the
default domain willy-nilly, so if it's a concern that drivers get stuck
having to use a translation domain if they do something dumb like:
if (!dma_set_mask(DMA_BIT_MASK(32))
dma_set_mask(DMA_BIT_MASK(64));
then the simple solution is "don't do that" - note that this doesn't
affect overriding of the default 32-bit mask, because we don't use the
driver API to initialise those.
> And we had a couple drivers playing
> interesting games there.
If the games you're worried about are stuff like:
dma_set_mask(dev, DMA_BIT_MASK(64));
high_buf = dma_alloc_coherent(dev, ...);
dma_set_mask(dev, DMA_BIT_MASK(32));
low_buf = dma_alloc_coherent(dev, ...);
then iommu_need_mapping() already ensures that will end spectacularly
badly. Unless we can somehow log when a mask has been "committed" by a
mapping operation, I don't think any kind of opportunistic bypass
mechanism is ever not going to blow up that case.
> FYI, this is the current intel-iommu
> WIP conversion to the dma bypass flag:
>
> http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-bypass
Having thought a bit more, I guess my idea does end up with one slightly
ugly corner wherein dma_direct_supported() has to learn to look for an
IOMMU default domain and try iommu_dma_supported() before saying no,
even if it's clean everywhere else. The bypass flag is more 'balanced'
in terms of being equally invasive everywhere and preserving abstraction
a bit better. Plus I think it might let us bring back the default
assignment of dma_dummy_ops, which I do like the thought of :D
Either way, making sure that the fundamental bypass decision is correct
and robust is still far more important than the implementation details.
Robin.
next prev parent reply other threads:[~2019-11-19 17:42 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-13 13:37 generic DMA bypass flag Christoph Hellwig
2019-11-13 13:37 ` [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device Christoph Hellwig
2019-11-13 13:37 ` [PATCH 2/2] powerpc: use the generic dma_ops_bypass mode Christoph Hellwig
2019-11-13 14:45 ` generic DMA bypass flag Robin Murphy
2019-11-14 7:41 ` Christoph Hellwig
2019-11-15 18:12 ` Robin Murphy
2019-11-16 6:22 ` Christoph Hellwig
2019-11-19 17:41 ` Robin Murphy [this message]
2019-11-20 11:16 ` Christoph Hellwig
2019-11-21 7:34 ` Christoph Hellwig
2019-11-21 16:44 ` Robin Murphy
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