linux-kernel - Re: [PATCH v2 4/7] iommu: Let iommu.strict override ops->def_domain

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Message-ID: <9163440eb6a47fe02730638bbdf72fda5ee5ad2c.camel@linux.ibm.com>
Date:   Mon, 28 Nov 2022 12:10:39 +0100
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Gerd Bayer <gbayer@...ux.ibm.com>, iommu@...ts.linux.dev,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Wenjia Zhang <wenjia@...ux.ibm.com>
Cc:     Pierre Morel <pmorel@...ux.ibm.com>, linux-s390@...r.kernel.org,
        borntraeger@...ux.ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, linux-kernel@...r.kernel.org,
        Julian Ruess <julianr@...ux.ibm.com>
Subject: Re: [PATCH v2 4/7] iommu: Let iommu.strict override
 ops->def_domain_type

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@...ux.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.

> 
> > +
> >   	if (dev_is_pci(dev) && to_pci_dev(dev)->untrusted)
> >   		return IOMMU_DOMAIN_DMA;
> >   
> 
> Best regards,
> baolu