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

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Date:   Fri, 25 Sep 2020 16:58:53 +0700
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     Robin Murphy <robin.murphy@....com>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 02/13] iommu: amd: Prepare for generic IO page table
 framework

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