Re: [PATCH v11 1/3] iommu: Enhance IOMMU default DMA mode build options

[John Garry]

On 09/06/2021 16:03, Robin Murphy wrote:
> On 2021-06-08 14:18, John Garry wrote:
> > From: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> >
> > First, add build options IOMMU_DEFAULT_{LAZY|STRICT}, so that we have the
> > opportunity to set {lazy|strict} mode as default at build time. Then put
> > the two config options in a choice, as they are mutually exclusive.
> >
> > [jpg: Make choice between strict and lazy only (and not passthrough)]
> > Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> > ---
> >   drivers/iommu/Kconfig | 35 +++++++++++++++++++++++++++++++++++
> >   drivers/iommu/iommu.c |  3 ++-
> >   2 files changed, 37 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> > index 1f111b399bca..369a3af9e5bf 100644
> > --- a/drivers/iommu/Kconfig
> > +++ b/drivers/iommu/Kconfig
> > @@ -90,6 +90,41 @@ config IOMMU_DEFAULT_PASSTHROUGH
> >         If unsure, say N here.
> > +choice
> > +    prompt "IOMMU default DMA mode"
> > +    depends on IOMMU_API
> > +    depends on X86 || IA64 || X86_64 || ARM || ARM64
>
> Simply "depends on IOMMU_DMA" should suffice, since that's now the only 
> place where flush queues matter.

I suppose so.

Configs ARM64, AMD_IOMMU, and INTEL_IOMMU all select this.

> > +
> > +    default IOMMU_DEFAULT_STRICT
> > +    help
> > +      This option allows an IOMMU DMA mode to be chosen at build 
> > time, to
> > +      override the default DMA mode of each ARCH, removing the need to
> > +      pass in kernel parameters through command line. It is still 
> > possible
> > +      to provide ARCH-specific or common boot options to override this
> > +      option.
> > +
> > +      If unsure, keep the default.
> > +
> > +config IOMMU_DEFAULT_LAZY
> > +    bool "lazy"
> > +    help
> > +      Support lazy mode, where for every IOMMU DMA unmap operation, the
> > +      flush operation of IOTLB and the free operation of IOVA are 
> > deferred.
> > +      They are only guaranteed to be done before the related IOVA 
> > will be
> > +      reused.
> > +
> > +config IOMMU_DEFAULT_STRICT
> > +    bool "strict"
> > +    help
> > +      For every IOMMU DMA unmap operation, the flush operation of 
> > IOTLB and
> > +      the free operation of IOVA are guaranteed to be done in the unmap
> > +      function.
> > +
> > +      This mode is safer than lazy mode, but it may be slower in some 
> > high
> > +      performance scenarios.
>
> FWIW, as an end user who doesn't care much about the implementation 
> details I'd probably appreciate the actual implications being clearer, 
> i.e. what does "safer" mean in practice and what is it relative to?

Fine, I can mention that lazy mode means that we have reduced device 
isolation and a dangerous window can be created between device driver 
DMA unmap and zapping the mapping in the IOMMU; however still much safer 
than passthrough/no IOMMU, which means no isolation at all.

Thanks,
John