RE: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

[Tian, Kevin]

> From: Tian, Kevin
> Sent: Thursday, September 23, 2021 11:11 AM
> 
> >
> > The required behavior for iommufd is to have the IOMMU ignore the
> > no-snoop bit so that Intel HW can disable wbinvd. This bit should be
> > clearly documented for its exact purpose and if other arches also have
> > instructions that need to be disabled if snoop TLPs are allowed then
> > they can re-use this bit. It appears ARM does not have this issue and
> > does not need the bit.
> 
> Disabling wbinvd is one purpose. imo the more important intention
> is that iommu vendor uses different PTE formats between snoop and
> !snoop. As long as we want allow userspace to opt in case of isoch
> performance requirement (unlike current vfio which always choose
> snoop format if available), such mechanism is required for all vendors.
> 

btw I'm not sure whether the wbinvd trick is Intel specific. All other
platforms (amd, arm, s390, etc.) currently always claim OMMU_CAP_
CACHE_COHERENCY (the source of IOMMU_CACHE). They didn't hit
this problem because vfio always sets IOMMU_CACHE to force every
DMA to snoop. Will they need to handle similar wbinvd-like trick (plus
necessary memory type virtualization) when non-snoop format is enabled? 
Or are their architectures highly optimized to afford isoch traffic even 
with snoop (then fine to not support user opt-in)?

Thanks
Kevin