Re: [PATCH v2] iommu/amd: Set translation valid bit only when IO page tables are in used

[Suravee Suthikulpanit via iommu <iommu@lists.linux-foundation.org>]

Joerg,

On 5/13/22 8:07 PM, Joerg Roedel wrote:
> On Mon, May 09, 2022 at 02:48:15AM -0500, Suravee Suthikulpanit wrote:
>> On AMD system with SNP enabled, IOMMU hardware checks the host translation
>> valid (TV) and guest translation valid (GV) bits in the device
>> table entry (DTE) before accessing the corresponded page tables.
>>
>> However, current IOMMU driver sets the TV bit for all devices
>> regardless of whether the host page table is in used.
>> This results in ILLEGAL_DEV_TABLE_ENTRY event for devices, which
>> do not the host page table root pointer set up.
> 
> Hmm, this sound weird. In the early AMD IOMMUs it was recommended to set
> TV=1 and V=1 and the rest to 0 to block all DMA from a device.
>
> I wonder how this triggers ILLEGAL_DEV_TABLE_ENTRY errors now. It is
> (was?) legal to set V=1 TV=1, mode=0 and leave the page-table empty.

Due to the new restriction (please see the IOMMU spec Rev 3.06-PUB - Apr 2021
https://www.amd.com/system/files/TechDocs/48882_IOMMU.pdf) where the use of
DTE[Mode]=0 is not supported on systems that are SNP-enabled (i.e. EFR[SNPSup]=1),
the IOMMU HW looks at the DTE[TV] bit to determine if it needs to handle the v1 page table.
When the HW encounters DTE entry with TV=1, V=1, Mode=0, it would generate
ILLEGAL_DEV_TABLE_ENTRY event.

Note: I am following up with HW folks for the updated document for this
specific detail.

Therefore, we need to modify IOMMU driver as following:

- For non-DMA devices (e.g. the IOAPIC devices), we need to
modify IOMMU driver to default to DTE[TV]=0. For Linux, this is equivalent
to DTE with domain ID 0.

- I am still trying to see what is the best way to force Linux to not allow
Mode=0 (i.e. iommu=pt mode). Any thoughts?

- Also, it seems that the current iommu v2 page table use case, where GVA->GPA=SPA
will no longer be supported on system w/ SNPSup=1. Any thoughts?

> When then IW=0 and IR=0, DMA is blocked. From what I remember this is a
> valid setting in a DTE.

Correct.

> Do you have an example DTE which triggers this error message?

This is specifically from the device representing an IOAPIC.

[  +0.000108] iommu ivhd0: AMD-Vi: Event logged [ILLEGAL_DEV_TABLE_ENTRY device=c0:00.1 pasid=0x00000 
address=0xfffffffdf8140000 flags=0x0008]
[  +0.000011] AMD-Vi: DTE[0]: 0000000000000003
[  +0.000003] AMD-Vi: DTE[1]: 0000000000000000
[  +0.000002] AMD-Vi: DTE[2]: 2008000100258013
[  +0.000001] AMD-Vi: DTE[3]: 0000000000000000

Best Regards,
Suravee
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