Re: [PATCH 3/3] iommu/uapi: Add helper function for size lookup

[Jean-Philippe Brucker]

On Mon, Feb 03, 2020 at 02:41:02PM -0800, Jacob Pan wrote:
> Yeah, that would work as well. I just feel IOMMU UAPI is unlikely to get
> updated frequently, should be much less than adding new capabilities.
> I think argsz could be viewed as the version field set by the
> user, minsz is what kernel current code supports.
> 
> So let me summarize the options we have
> 1. Disallow adding new members to each structure other than reuse
> padding bits or adding union members at the end.
> 2. Allow extension of the structures beyond union, but union size has
> to be fixed with reserved spaces
> 3. Adopt VFIO argsz scheme, I don't think we need version for each
> struct anymore. argsz implies the version that user is using assuming
> UAPI data is extension only.
> 
> Jean, Eric, any comments? My preference is #1. In the apocalyptic event
> when we run out of padding, perhaps we can introduce a new API_v2 :)

I agree, new extensions will most likely want to extend the vendor
specific structures at the end rather than introduce new common fields, so
I prefer #1 which avoids fixing the union size.

Thanks,
Jean