Re: [RFC 0/7] Proposal to make x86 IOMMU driver support configurable

[Xenia Ragiadakou <burzalodowa@xxxxxxxxx>]

Hi,

On 12/19/22 20:28, Andrew Cooper wrote:
> On 19/12/2022 6:34 am, Xenia Ragiadakou wrote:
> > This series aims to provide a means to render the iommu driver support for x86
> > configurable. Currently, irrespectively of the target platform, both AMD and
> > Intel iommu drivers are built. This is the case because the existent Kconfig
> > infrastructure does not provide any facilities for finer-grained configuration.
> >
> > The series adds two new Kconfig options, AMD_IOMMU and INTEL_VTD, that can be
> > used to generate a tailored iommu configuration for a given platform.
> >
> > This series will be part of a broader effort to separate platform specific
> > code and it is sent as an RFC to gather feedback regarding the way the
> > separation should be performed.
> Hello,
>
> Thanks for the series.
>
>  From discussions in the past, we do want CONFIG_INTEL/AMD/etc and we do
> want these to be selectable and available for randconfig to test.
>
> Some sub-features are more complicated, because e.g. Centaur have CPUs
> with a VT-x implementation.  This will need expressing in whatever
> Kconfig scheme we end up with.
What about having configuration per cpu vendor, per virtualization 
technology and per IOMMU technology? I mean sth like [CPU_AMD, 
CPU_HYGON, CPU_INTEL, CPU_SHANGHAI, CPU_CENTAUR], [AMD_SVM, INTEL_VMX] 
and [AMD_IOMMU, INTEL_IOMMU], respectively?
> IOMMUs are more tricky still.  They are (for most intents and purposes)
> mandatory on systems with >254 CPUs.  We could in principle start
> supporting asymmetric IRQ routing on large systems, but Xen doesn't
> currently and it would be a lot work that's definitely not high on the
> priority list.  At a minimum, this will need expressing in the Kconfig
> help text.
Ok.

> We need to decide whether it is sensible to allow users to turn off
> IOMMU support.  It probably is, because you could trivially do this by
> selecting CONFIG_INTEL only, and booting the result on an AMD system.
I cannot understand. I guess that if the code for the target system is 
disabled, Xen won't boot ... hopefully :).
If users are not allowed to turn off the IOMMU support, it can be always 
enabled unconditionally via Kconfig select based on the virtualization 
technology or other.
> For the names, I don't think AMD_IOMMU vs INTEL_VTD is a sensible.
> Probably wants to be INTEL_IOMMU to match.
Ok.

> ~Andrew
--
Xenia