Re: [RFC PATCH 1/3] iommu: Add support for out of band flushing

From: Thierry Reding
Date: Tue Sep 29 2015 - 05:33:20 EST


On Tue, Sep 29, 2015 at 02:25:24PM +0900, Tomasz Figa wrote:
> This patch adds a new "flush" callback to iommu_ops, which is supposed
> to perform any necessary flushes within given IOMMU domain to make any
> changes to mappings of given area [iova; iova + size) be reflected to
> IOMMU clients.
>
> The purpose is to let IOMMU drivers skip page-by-page flushes and
> replace it with one flush of full address range on devices which support
> it.
>
> Signed-off-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> ---
> drivers/iommu/iommu.c | 33 ++++++++++++++++++++++++++++++---
> include/linux/iommu.h | 2 ++
> 2 files changed, 32 insertions(+), 3 deletions(-)

I seem to remember that Rob Clark had proposed something like this back
before it was decided to introduce the ->map_sg() callback instead. I
can't find a reference to the discussion, so perhaps I'm misremembering
but adding Rob Clark in case he has any recollection of why the outcome
was what it was.

> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
[...]
> @@ -1389,6 +1410,7 @@ size_t iommu_unmap(struct iommu_domain *domain, unsigned long iova, size_t size)
> unmapped += unmapped_page;
> }
>
> + iommu_flush(domain, orig_iova, unmapped);
> trace_unmap(orig_iova, size, unmapped);
> return unmapped;
> }
> @@ -1419,19 +1441,24 @@ size_t default_iommu_map_sg(struct iommu_domain *domain, unsigned long iova,
> if (!IS_ALIGNED(s->offset, min_pagesz))
> goto out_err;
>
> - ret = iommu_map(domain, iova + mapped, phys, s->length, prot);
> + ret = __iommu_map(domain, iova + mapped, phys, s->length, prot);
> if (ret)
> goto out_err;
>
> mapped += s->length;
> }
>
> + iommu_flush(domain, iova, mapped);
> +
> return mapped;
>
> out_err:
> /* undo mappings already done */
> iommu_unmap(domain, iova, mapped);
>
> + /* flush in case part of our mapping already got cached */
> + iommu_flush(domain, iova, mapped);
> +
> return 0;
>
> }

iommu_unmap() already does an iommu_flush(), so why flush again after
iommu_unmap()?

Thierry

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