Re: [git pull] IOMMU Updates for Linux v5.15

[Robin Murphy]

On 2021-09-03 22:44, Joerg Roedel wrote:
> On Fri, Sep 03, 2021 at 11:43:31AM -0700, Linus Torvalds wrote:
> >    choice
> >          prompt "IOMMU default domain type"
> >          depends on IOMMU_API
> >          default IOMMU_DEFAULT_DMA_LAZY if AMD_IOMMU || INTEL_IOMMU
> >          default IOMMU_DEFAULT_DMA_STRICT
>
> Huh, yeah, that is bogus. Seems like I overlooked that part of the
> patch-set because I was so amazed by the simplifications and cleanups in
> the rest of it.

Mad as it looks, this does in fact capture the existing behaviour. What 
we've consolidated here was previously a weird mix of driver- and 
subsystem-level controls, and it is specifically those two drivers which 
have a long-standing expectation of using lazy behaviour by default.

> > See what I'm saying? Making the default be based on some random "this
> > driver is enabled" when it can then affect *other* drivers that are
> > also enabled and not part of the decision seems to be a fundamental
> > confusion about what is going on, when it's not at all clear which
> > driver will actually be IN USE.
>
> The Kconfig option itself was actually my suggestion, but how the
> default value is chosen certainly needs improvement. I will sort this
> out with the people involved.
>
> > IOW, the fix might be to just say "the default is always lazy".
> >
> > Or the fix might be something that is global to a configuration and
> > doesn't rely on which iommu is in use (eg "on x86, the default is
> > always LAZY")

We could certainly express it as "default IOMMU_DEFAULT_DMA_LAZY if X86" 
if people would prefer - virtio-iommu doesn't support lazy mode either 
way, so the end result will still be equivalent.

Robin.

> > Or the fix is to make that 'iommu_dma_strict' variable - and the
> > default value for it - be a per-IOMMU thing rather than be a global.
>
> My preference would be to make 'lazy' or 'strict' the default for all,
> but the ARM folks might disagree. On the other side it also doesn't make
> sense to let IOMMU drivers override the users Kconfig choice at runtime.
> We will discuss this and come up with something better. >
> Thanks,
>
> 	Joerg
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