linux-kernel - Re: [PATCH 1/4] iommu/amd: Introduce Protection-domain flag VFIO

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Date:   Fri, 20 Jan 2023 13:55:38 -0600
From:   "Kalra, Ashish" <ashish.kalra@....com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
        joro@...tes.org, robin.murphy@....com, thomas.lendacky@....com,
        vasant.hegde@....com, jon.grimm@....com
Subject: Re: [PATCH 1/4] iommu/amd: Introduce Protection-domain flag VFIO

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
>