Re: generic DMA bypass flag
From: Robin Murphy
Date: Tue Nov 19 2019 - 12:42:02 EST
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:
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:
high_buf = dma_alloc_coherent(dev, ...);
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:
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.