Re: [PATCH] iommu: Allow ATS to work on VFs when the PF uses IDENTITY

[Yi Liu <yi.l.liu@intel.com>]

On 2024/8/12 19:35, Jason Gunthorpe wrote:
> On Mon, Aug 12, 2024 at 05:03:37PM +0800, Yi Liu wrote:
> 
>>> The PASID may require ATS to be enabled (ie SVA), but the RID may be
>>> IDENTITY for performance. The poor device has no idea it is not
>>> allowed to use ATS on the RID side :(
>>
>> If this is the only problematic case, the intel iommu driver in this
>> patch could check the scalable mode before enabling ATS in the
>> probe_device() op. In this way, the legacy mode iommu would keep the old
>> ATS enable policy.
> 
> At some point we will need to address this.. At least when we add
> PASID support to mlx5 it will need something since mlx5 is using ATS
> on the RID right now.
> 
> Supporting ATS with the IDENTITY optimization is a good idea in the
> IOMMU HW. The HW just has to answer the ATS with a 1:1 TA.

looks like Intel scalable mode would allow ATS work even device uses
IDENTITY. Below is what spec says when PGTT is PT.

Chapter 9.6:
Untranslated/Translation Requests (with or without PASID) referencing this
scalable-mode PASID Table Entry bypass address translation and are
processed as pass-through.

> Next, telling the driver that ATS is enabled but doesn't work on the
> RID would allow the driver to deal with it
> 
> Finally, doing things like using IDENTITY through a paging domain
> would be a last resort for troubled devices.

yep. It's another issue then.

-- 
Regards,
Yi Liu