Re: [RFC PATCH 0/3] iommu: Add range flush operation

[Robin Murphy]

On 29/09/15 17:40, Russell King - ARM Linux wrote:
> On Tue, Sep 29, 2015 at 05:27:12PM +0100, Robin Murphy wrote:
> > Eh, swings and roundabouts. An argument denoting whether the flush is being
> > called on the map or unmap path would be fine,
>
> Sorry, that statement is wrong.  It's not about whether you flush before
> or after the DMA operation.  I'm afraid I'm probably going to tell you
> how to suck eggs here, because I don't think you quite "get it" with
> non-dma-coherent modern CPUs.
>
> Modern CPUs prefetch data into their caches, and they also randomly write
> back data from their caches to memory.  When performing a DMA operation
> from device to memory, you need to do two things with CPU caches which
> aren't coherent:
>
> 1. Before starting the DMA operation, you need to walk over the memory to
>     be mapped, ensuring that any dirty cache lines are written back.  This
>     is to prevent dirty cache lines overwriting data that has already been
>     DMA'd from the device.
>
> 2. After the DMA operation has completed, you need to walk over the
>     memory again, invalidating any cache lines which may have been
>     speculatively loaded from that memory while DMA was running.  These
>     cache lines may have been loaded prior to the DMA operation placing
>     the new data into memory.
>
> So, it's not a before-or-after, you have to always perform write-back
> cache maintanence prior to any DMA operation, and then invalidate cache
> maintanence after the DMA operation has completed for any mapping which
> the DMA may have written to (which means device-to-memory and
> bidirectional mappings.)

Yup, I'm well aware of all that; in fact you and I have already agreed 
elsewhere that we can only really get away with using the streaming DMA 
API to flush IOMMU page table updates _because_ they aren't written back 
to, thus data only ever goes from CPU->IOMMU and we can skip the problem 
of where to put an invalidation; you wrote the tegra-smmu code that does 
this. The coherency of whatever device which made a DMA API call for 
which the IOMMU API is creating/removing a mapping is irrelevant at this 
point - this is the DMA operation within the DMA operation.

None of which has anything to do with the point I raised, which is that 
if iommu_unmap() calls iommu_flush(), I want to issue TLB invalidations, 
but if iommu_map() calls iommu_flush(), I don't.

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