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From: David Stevens <stevensd@chromium.org>
To: Will Deacon <will@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>,
	Joerg Roedel <joro@8bytes.org>,
	Lu Baolu <baolu.lu@linux.intel.com>, Tom Murphy <murphyt7@tcd.ie>,
	iommu@lists.linux-foundation.org,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 1/4] dma-iommu: fix sync_sg with swiotlb
Date: Thu, 5 Aug 2021 12:40:40 +0900	[thread overview]
Message-ID: <CAD=HUj6BoCZ31Dd-jeWKKbmt-jSWXiS2ZLwrFg40FFdPm0y7cA@mail.gmail.com> (raw)
In-Reply-To: <20210802133037.GB28547@willie-the-truck>

On Mon, Aug 2, 2021 at 10:30 PM Will Deacon <will@kernel.org> wrote:
>
> On Fri, Jul 09, 2021 at 12:34:59PM +0900, David Stevens wrote:
> > From: David Stevens <stevensd@chromium.org>
> >
> > The is_swiotlb_buffer function takes the physical address of the swiotlb
> > buffer, not the physical address of the original buffer. The sglist
> > contains the physical addresses of the original buffer, so for the
> > sync_sg functions to work properly when a bounce buffer might have been
> > used, we need to use iommu_iova_to_phys to look up the physical address.
> > This is what sync_single does, so call that function on each sglist
> > segment.
> >
> > The previous code mostly worked because swiotlb does the transfer on map
> > and unmap. However, any callers which use DMA_ATTR_SKIP_CPU_SYNC with
> > sglists or which call sync_sg would not have had anything copied to the
> > bounce buffer.
> >
> > Fixes: 82612d66d51d ("iommu: Allow the dma-iommu api to use bounce buffers")
> > Signed-off-by: David Stevens <stevensd@chromium.org>
> > ---
> >  drivers/iommu/dma-iommu.c | 26 +++++++++++++-------------
> >  1 file changed, 13 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > index 7bcdd1205535..eac65302439e 100644
> > --- a/drivers/iommu/dma-iommu.c
> > +++ b/drivers/iommu/dma-iommu.c
> > @@ -811,14 +811,14 @@ static void iommu_dma_sync_sg_for_cpu(struct device *dev,
> >       if (dev_is_dma_coherent(dev) && !dev_is_untrusted(dev))
> >               return;
> >
> > -     for_each_sg(sgl, sg, nelems, i) {
> > -             if (!dev_is_dma_coherent(dev))
> > -                     arch_sync_dma_for_cpu(sg_phys(sg), sg->length, dir);
> > -
> > -             if (is_swiotlb_buffer(sg_phys(sg)))
> > +     if (dev_is_untrusted(dev))
> > +             for_each_sg(sgl, sg, nelems, i)
> > +                     iommu_dma_sync_single_for_cpu(dev, sg_dma_address(sg),
> > +                                                   sg->length, dir);
> > +     else
> > +             for_each_sg(sgl, sg, nelems, i)
> >                       swiotlb_sync_single_for_cpu(dev, sg_phys(sg),
> >                                                   sg->length, dir);
>
> Doesn't this skip arch_sync_dma_for_cpu() for non-coherent trusted devices?

Whoops, this was supposed to be a call to arch_sync_dma_for_cpu, not
to swiotlb_sync_single_for_cpu. Similar to the sync_sg_for_device
case.

> Why not skip the extra dev_is_untrusted(dev) call here and just call
> iommu_dma_sync_single_for_cpu() for each entry regardless?

iommu_dma_sync_single_for_cpu calls iommu_iova_to_phys to translate
the dma_addr_t to a phys_addr_t. Since the physical address is readily
available, I think it's better to avoid that extra work.

> Will

  reply	other threads:[~2021-08-05  3:40 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-09  3:34 [PATCH v2 0/4] Fixes for dma-iommu swiotlb bounce buffers David Stevens
2021-07-09  3:34 ` [PATCH v2 1/4] dma-iommu: fix sync_sg with swiotlb David Stevens
2021-08-02 13:30   ` Will Deacon
2021-08-05  3:40     ` David Stevens [this message]
2021-07-09  3:35 ` [PATCH v2 2/4] dma-iommu: fix arch_sync_dma for map " David Stevens
2021-08-02 13:40   ` Will Deacon
2021-08-02 13:50     ` Will Deacon
2021-07-09  3:35 ` [PATCH v2 3/4] dma-iommu: pass SKIP_CPU_SYNC to swiotlb unmap David Stevens
2021-08-02 13:54   ` Will Deacon
2021-08-05  5:26     ` David Stevens
2021-08-06 11:38       ` Will Deacon
2021-07-09  3:35 ` [PATCH v2 4/4] dma-iommu: Check CONFIG_SWIOTLB more broadly David Stevens

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