Re: [PATCH 3/8] iommu/vt-d: Require CMPXCHG16B for PASID support

[Jason Gunthorpe]

On Mon, Mar 09, 2026 at 02:06:43PM +0800, Lu Baolu wrote:
> The Intel IOMMU driver is moving toward using the generic entry_sync
> library for PASID table entry updates. This library requires 128-bit
> atomic write operations (cmpxchg128) to update 512-bit PASID entries in
> atomic quanta, ensuring the hardware never observes a torn entry.
> 
> On x86_64, 128-bit atomicity is provided by the CMPXCHG16B instruction.
> Update the driver to:
> 
> 1. Limit INTEL_IOMMU to X86_64, as 128-bit atomic operations are not
>    available on 32-bit x86.
> 2. Gate pasid_supported() on the presence of X86_FEATURE_CX16.
> 3. Provide a boot-time warning if a PASID-capable IOMMU is detected on
>    a CPU lacking the required instruction.

This is fine, but it also occured to me that we could change the
writer somewhat to just detect what the update granual is and fall
back to 64 bit in this case. So everything still works, it just does
non-present alot more often.

Jason