Re: [PATCH v4 01/17] iommu: Add hwpt_type with user_data for domain_alloc_user op

[Robin Murphy]

On 2023-09-21 17:44, Jason Gunthorpe wrote:
> On Thu, Sep 21, 2023 at 08:12:03PM +0800, Baolu Lu wrote:
> > On 2023/9/21 15:51, Yi Liu wrote:
> > > diff --git a/include/uapi/linux/iommufd.h b/include/uapi/linux/iommufd.h
> > > index 4a7c5c8fdbb4..3c8660fe9bb1 100644
> > > --- a/include/uapi/linux/iommufd.h
> > > +++ b/include/uapi/linux/iommufd.h
> > > @@ -357,6 +357,14 @@ enum iommufd_hwpt_alloc_flags {
> > >    	IOMMU_HWPT_ALLOC_NEST_PARENT = 1 << 0,
> > >    };
> > > +/**
> > > + * enum iommu_hwpt_type - IOMMU HWPT Type
> > > + * @IOMMU_HWPT_TYPE_DEFAULT: default
> >
> > How about s/default/vendor agnostic/ ?
>
> Please don't use the word vendor :)
>
> IOMMU_HWPT_TYPE_GENERIC perhaps if we don't like default

Ah yes, a default domain type, not to be confused with any default 
domain type, including the default default domain type. Just in case 
anyone had forgotten how gleefully fun this is :D

I particularly like the bit where we end up with this construct later:

	switch (hwpt_type) {
	case IOMMU_HWPT_TYPE_DEFAULT:
		/* allocate a domain */
	default:
		/* allocate a different domain */
	}

But of course neither case allocates a *default* domain, because it's 
quite obviously the wrong place to be doing that.

I could go on enjoying myself, but basically yeah, "default" can't be a 
type in itself (at best it would be a meta-type which could be 
requested, such that it resolves to some real type to actually 
allocate), so a good name should reflect what the type functionally 
*means* to the user. IIUC the important distinction is that it's an 
abstract kernel-owned pagetable for the user to indirectly control via 
the API, rather than one it owns and writes directly (and thus has to be 
in a specific agreed format).

Thanks,
Robin.