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Date:   Mon, 28 Nov 2022 09:29:51 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Niklas Schnelle <schnelle@linux.ibm.com>
Cc:     Baolu Lu <baolu.lu@linux.intel.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        Gerd Bayer <gbayer@linux.ibm.com>, iommu@lists.linux.dev,
        Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Wenjia Zhang <wenjia@linux.ibm.com>,
        Pierre Morel <pmorel@linux.ibm.com>,
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        svens@linux.ibm.com, linux-kernel@vger.kernel.org,
        Julian Ruess <julianr@linux.ibm.com>
Subject: Re: [PATCH v2 4/7] iommu: Let iommu.strict override
 ops->def_domain_type
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On Mon, Nov 28, 2022 at 12:10:39PM +0100, Niklas Schnelle wrote:
> On Thu, 2022-11-17 at 09:55 +0800, Baolu Lu wrote:
> > On 2022/11/17 1:16, Niklas Schnelle wrote:
> > > When iommu.strict=1 is set or iommu_set_dma_strict() was called we
> > > should use IOMMU_DOMAIN_DMA irrespective of ops->def_domain_type.
> > > 
> > > Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> > > ---
> > >   drivers/iommu/iommu.c | 3 +++
> > >   1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > > index 65a3b3d886dc..d9bf94d198df 100644
> > > --- a/drivers/iommu/iommu.c
> > > +++ b/drivers/iommu/iommu.c
> > > @@ -1562,6 +1562,9 @@ static int iommu_get_def_domain_type(struct device *dev)
> > >   {
> > >   	const struct iommu_ops *ops = dev_iommu_ops(dev);
> > >   
> > > +	if (iommu_dma_strict)
> > > +		return IOMMU_DOMAIN_DMA;
> > 
> > If any quirky device must work in IOMMU identity mapping mode, this
> > might introduce functional regression. At least for VT-d platforms, some
> > devices do require IOMMU identity mapping mode for functionality.
> 
> That's a good point. How about instead of unconditionally returning
> IOMMU_DOMAIN_DMA we just do so if the domain type returned by ops-
> >def_domain_type uses a flush queue (i.e. the __IOMMU_DOMAIN_DMA_FQ bit
> is set). That way a device that only supports identity mapping gets to
> set that but iommu_dma_strict at least always prevents use of an IOVA
> flush queue.

I would prefer we create some formal caps in iommu_ops to describe
whatever it is you are trying to do.

Jason