lkml.org 
[lkml]   [2015]   [Sep]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC PATCH 0/3] iommu: Add range flush operation
From
Date
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.



\
 
 \ /
  Last update: 2015-09-29 19:41    [W:0.053 / U:0.312 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site