Re: [PATCH 02/13] iommu: amd: Prepare for generic IO page table framework

[Suravee Suthikulpanit]

Robin,

On 9/24/20 7:25 PM, Robin Murphy wrote:
> > +struct io_pgtable_ops *amd_iommu_setup_io_pgtable_ops(struct iommu_dev_data *dev_data,
> > +                         struct protection_domain *domain)
> > +{
> > +        domain->iop.pgtbl_cfg = (struct io_pgtable_cfg) {
> > +        .pgsize_bitmap    = AMD_IOMMU_PGSIZES,
> > +        .ias        = IOMMU_IN_ADDR_BIT_SIZE,
> > +        .oas        = IOMMU_OUT_ADDR_BIT_SIZE,
> > +        .coherent_walk    = false,
>
> Is that right? Given that you seem to use regular kernel addresses for pagetable pages and don't have any obvious cache 
> maintenance around PTE manipulation, I suspect not ;)
> > It's fair enough if your implementation doesn't use this and simply assumes coherency, but in that case it would be less
> confusing to have the driver set it to true for the sake of honesty, or just leave it out 
> entirely - explicitly setting false gives the illusion of being meaningful.

AMD IOMMU can be configured to disable snoop for page table walk of a particular device (DTE[SD]=1). However, the 
current Linux driver does not set this bit, which should assume coherency. We can just leaving this out for now. I can 
remove this when I send out V2 along w/ other changes.

> Otherwise, the io-pgtable parts all look OK to me - it's nice to finally 
> fulfil the original intent of not being an Arm-specific thing :D
>
> Robin.

Thanks,
Suravee