Re: [PATCH v3 2/5] dma-iommu: fix arch_sync_dma for map

[Robin Murphy]

On 2021-08-12 10:21, David Stevens wrote:
> On Thu, Aug 12, 2021 at 3:47 AM Robin Murphy <robin.murphy@xxxxxxx> wrote:
> > On 2021-08-11 03:42, David Stevens wrote:
> > > From: David Stevens <stevensd@xxxxxxxxxxxx>
> > >
> > > When calling arch_sync_dma, we need to pass it the memory that's
> > > actually being used for dma. When using swiotlb bounce buffers, this is
> > > the bounce buffer. Move arch_sync_dma into the __iommu_dma_map_swiotlb
> > > helper, so it can use the bounce buffer address if necessary. This also
> > > means it is no longer necessary to call iommu_dma_sync_sg_for_device in
> > > iommu_dma_map_sg for untrusted devices.
> > >
> > > Fixes: 82612d66d51d ("iommu: Allow the dma-iommu api to use bounce buffers")
> > > Signed-off-by: David Stevens <stevensd@xxxxxxxxxxxx>
> > > ---
> > >    drivers/iommu/dma-iommu.c | 16 +++++++---------
> > >    1 file changed, 7 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > > index 54e103b989d9..4f0cc4a0a61f 100644
> > > --- a/drivers/iommu/dma-iommu.c
> > > +++ b/drivers/iommu/dma-iommu.c
> > > @@ -576,6 +576,9 @@ static dma_addr_t __iommu_dma_map_swiotlb(struct device *dev, phys_addr_t phys,
> > >                memset(padding_start, 0, padding_size);
> > >        }
> > >
> > > +     if (!coherent && !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> >
> > Make that an "else if" - otherwise you're just reintroducing the same
> > thing that the third hunk is trying to clean up.
>
> swiotlb_tbl_map_single only copies into the swiotlb buffer, it doesn't
> do any architectural syncing. So this block is necessary in the
> swiotlb case as well, for iommu_dma_map_page to work properly.

Urgh, apologies again - I had a mental short-circuit and was convinced 
that the SWIOTLB call already did its own cache maintenance. I really 
should have learned by now not to review fiddly patches like these while 
tired...
> The third chunk is a separate optimization, so I'll split it out into
> its own patch.

It's still a logical part of this change - cleaning up the potentially 
wrong sync while you make sure the right one happens. Even more so if 
you make the fix the better way by having iommu_dma_map_sg_swiotlb() 
call iommu_dma_map_page().

Robin.