linux-kernel - Re: [PATCH 9/9] iommu/amd: Set default domain to IDENTITY

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Date: Mon, 13 May 2024 20:10:27 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
Cc: linux-kernel@...r.kernel.org, iommu@...ts.linux.dev, joro@...tes.org,
	thomas.lendacky@....com, vasant.hegde@....com, michael.roth@....com,
	jon.grimm@....com, rientjes@...gle.com
Subject: Re: [PATCH 9/9] iommu/amd: Set default domain to IDENTITY_DOMAIN
 when running in SEV guest

On Mon, May 13, 2024 at 07:17:49PM +0700, Suthikulpanit, Suravee wrote:
> Jason,
> 
> On 5/1/2024 9:17 PM, Jason Gunthorpe wrote:
> > On Tue, Apr 30, 2024 at 03:24:30PM +0000, Suravee Suthikulpanit wrote:
> > > Since SEV guest depends on the unencrypted swiotlb bounce buffer
> > > to support DMA, the guest AMD IOMMU driver must be force to setup to
> > > pass-through mode.
> > 
> > You should block the creation of paging domains as well if the HW
> > can't support them.
> 
> Sure, I'll add a logic to check and block domain creation.
> 
> > But, is there actually a functional problem here? Doesn't swiotlb work
> > OK with iommu even with the encrypted memory cases? What is missing if
> > not?
> 
> Currently, SEV guest is default to use SWIOTLB. This does not have any
> issues.
> 
> However, in order to support vcpus w/ x2APIC ID (> 255) in a guest, it
> requires guest interrupt remapping support. This is achieved by adding
> QEMU-emulated AMD or Intel vIOMMU models.
> 
> In case of AMD IOMMU, depending on the CONFIG_IOMMU_DEFAULT_PASSTHROUGH
> kernel config, it would default to setup the v1 table for DMA remapping,
> which is not supported in the SEV guest (since it requires to use SWIOTLB).

But this just means you are inserting an iommu hw that is totally
non-working. I'd expect that the iommu continues to work correctly but
cannot access any encrypted pages..

If it is unusable do you even need to allow it to probe to any
drivers? Nothing works so there isn't much point to binding devices to
the iommu..?

Jason