linux-kernel - Re: [PATCH v3 2/5] dma-iommu: fix arch_sync

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Message-ID: <CAD=HUj7RLtRkKwRHOAeuJXiiLp9WujOysYAoU0s2Nx5qR1_B7g@mail.gmail.com>
Date:   Thu, 12 Aug 2021 18:21:16 +0900
From:   David Stevens <stevensd@...omium.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Will Deacon <will@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Tom Murphy <murphyt7@....ie>, iommu@...ts.linux-foundation.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/5] dma-iommu: fix arch_sync_dma for map

On Thu, Aug 12, 2021 at 3:47 AM Robin Murphy <robin.murphy@....com> wrote:
>
> On 2021-08-11 03:42, David Stevens wrote:
> > From: David Stevens <stevensd@...omium.org>
> >
> > 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@...omium.org>
> > ---
> >   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.

The third chunk is a separate optimization, so I'll split it out into
its own patch.

-David