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SubjectRe: [PATCH 1/4] iommu/amd: Introduce Protection-domain flag VFIO
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On 1/20/2023 11:50 AM, Jason Gunthorpe wrote:
> On Fri, Jan 20, 2023 at 11:01:21AM -0600, Kalra, Ashish wrote:
>
>> We basically get the RMP #PF from the IOMMU because there is a page size
>> mismatch between the RMP table and the IOMMU page table. The RMP table's
>> large page entry has been smashed to 4K PTEs to handle page state change to
>> shared on 4K mappings, so this change has to be synced up with the IOMMU
>> page table, otherwise there is now a page size mismatch between RMP table
>> and IOMMU page table which causes the RMP #PF.
>
> I understand that, you haven't answered my question:
>
> Why is the IOMMU being programmed with pages it cannot access in the
> first place?
>

I believe the IOMMU page tables are setup as part of device pass-through
to be able to do DMA to all of the guest memory, but i am not an IOMMU
expert, so i will let Suravee elaborate on this.

Thanks,
Ashish

> Don't do that is the obvious solution there, and preserves huge page
> IO performance.
>
> Jason
>

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