linux-kernel - Re: [PATCH V5 3/5] iommu/dma-iommu: Handle deferred devices

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Date:   Sat, 17 Aug 2019 08:19:33 +0100
From:   Tom Murphy <murphyt7@....ie>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Joerg Roedel <joro@...tes.org>, iommu@...ts.linux-foundation.org,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Andy Gross <agross@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Rob Clark <robdclark@...il.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-tegra@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH V5 3/5] iommu/dma-iommu: Handle deferred devices

On Sat, 17 Aug 2019 at 04:39, Hillf Danton <hdanton@...a.com> wrote:
>
>
> On Thu, 15 Aug 2019 12:09:41 +0100 Tom Murphy wrote:
> >
> > Handle devices which defer their attach to the iommu in the dma-iommu api
> >
> > Signed-off-by: Tom Murphy <murphyt7@....ie>
> > ---
> >  drivers/iommu/dma-iommu.c | 27 ++++++++++++++++++++++++++-
> >  1 file changed, 26 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > index 2712fbc68b28..906b7fa14d3c 100644
> > --- a/drivers/iommu/dma-iommu.c
> > +++ b/drivers/iommu/dma-iommu.c
> > @@ -22,6 +22,7 @@
> >  #include <linux/pci.h>
> >  #include <linux/scatterlist.h>
> >  #include <linux/vmalloc.h>
> > +#include <linux/crash_dump.h>
> >
> >  struct iommu_dma_msi_page {
> >       struct list_head        list;
> > @@ -351,6 +352,21 @@ static int iommu_dma_init_domain(struct iommu_domain *domain, dma_addr_t base,
> >       return iova_reserve_iommu_regions(dev, domain);
> >  }
> >
> > +static int handle_deferred_device(struct device *dev,
> > +     struct iommu_domain *domain)
> > +{
> > +     const struct iommu_ops *ops = domain->ops;
> > +
> > +     if (!is_kdump_kernel())
> > +             return 0;
> > +
> > +     if (unlikely(ops->is_attach_deferred &&
> > +             ops->is_attach_deferred(domain, dev)))
> > +             return iommu_attach_device(domain, dev);
> > +
> > +     return 0;
> > +}
> > +
> >  /**
> >   * dma_info_to_prot - Translate DMA API directions and attributes to IOMMU API
> >   *                    page flags.
> > @@ -463,6 +479,9 @@ static dma_addr_t __iommu_dma_map(struct device *dev, phys_addr_t phys,
> >       size_t iova_off = iova_offset(iovad, phys);
> >       dma_addr_t iova;
> >
> > +     if (unlikely(handle_deferred_device(dev, domain)))
> > +             return DMA_MAPPING_ERROR;
> > +
> >       size = iova_align(iovad, size + iova_off);
> >
> >       iova = iommu_dma_alloc_iova(domain, size, dma_get_mask(dev), dev);
>
> iommu_map_atomic() is applied to __iommu_dma_map() in 2/5.
> Is it an atomic context currently given the mutex_lock() in
> iommu_attach_device()?

I don't see your point here. __iommu_dma_map isn't called from
iommu_attach_device, why would we care about a mutex in
iommu_attach_device?

__iommu_dma_map can be called from an atomic context (it isn't always
but it does happen). __iommu_dma_map is called by iommu_dma_alloc
which implements the iommu_dma_ops::alloc function which by design
needs to be callable from an atomic context. Does that answer your
question?

>