Re: [PATCH v2 1/2] iommu/amd: Add IOMMU_PROT_IE flag for memory encryption

[Vasant Hegde]

Wei,


On 11/11/2025 6:48 AM, Wei Wang wrote:
> On Monday, November 10, 2025 5:55 PM, Vasant Hegde wrote:
>> To: Wei Wang <wei.w.wang@xxxxxxxxxxx>; alex@xxxxxxxxxxx;
>> jgg@xxxxxxxxxx; suravee.suthikulpanit@xxxxxxx;
>> thomas.lendacky@xxxxxxx; joro@xxxxxxxxxx
>> Cc: kevin.tian@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>> iommu@xxxxxxxxxxxxxxx
>> Subject: Re: [PATCH v2 1/2] iommu/amd: Add IOMMU_PROT_IE flag for
>> memory encryption
>>
>> Hi Wei,
>>
>> On 11/3/2025 7:30 PM, Wei Wang wrote:
>>> Introduce the IOMMU_PROT_IE flag to allow callers of
>>> iommu_v1_map_pages() to explicitly request memory encryption for
>> specific mappings.
>>>
>>> With SME enabled, the C-bit (encryption bit) in IOMMU page table
>>> entries is now set only when IOMMU_PROT_IE is specified. This provides
>>> fine-grained control over which IOVAs are encrypted through the IOMMU
>>> page tables.
>>>
>>> Current PCIe devices and switches do not interpret the C-bit, so
>>> applying it to MMIO mappings would break PCIe peer‑to‑peer
>>> communication. Update the implementation to restrict C-bit usage to
>> non‑MMIO backed IOVAs.
>>
>> Right. Quote from AMD Programmers Manual Vol2, "any pages
>> corresponding to MMIO addresses must be configured with the C-bit clear."
>>
> 
> Yes, thanks.
> 
>>>
>>> Fixes: 2543a786aa25 ("iommu/amd: Allow the AMD IOMMU to work with
>>> memory encryption")
>>> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
>>> Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxxxx>
>>> --->  drivers/iommu/amd/amd_iommu_types.h | 3 ++-
>>>  drivers/iommu/amd/io_pgtable.c      | 7 +++++--
>>
>> May be apply same fix for io_pgtable_v2.c as well? (Of course filename
>> changed with generic pt series).
> 
> Yes. I was uncertain about the 1st stage mapping as it has a usage for
> GVA->GPA mappings, and for the trusted MMIO case, we do need the
> C-bit added to GPA. But since vIOMMU isn’t supported for SNP guests,
> and the trusted MMIO isn't ready yet, I think it should be safe to proceed
> with this now. The above consideration can be re-visited when the trusted
> MMIO gets landed.

Right. We use v2 (guest) page table to support PASID/PRI in host.

Agree. for now lets fix the v2 page table as well. We will fix it for Secure
vIOMMU when we add support.




> 
> I'll add it in the next version and see if others might have a different
> perspective on this.

Thanks!

-Vasant