Re: [PATCH v5 05/21] vfio/iommufd: Force creating nested parent domain

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Aug 29, 2025 at 01:40:01AM +0000, Duan, Zhenzhong wrote:
> >-----Original Message-----
> >On 8/28/25 11:53 AM, Duan, Zhenzhong wrote:
> >>>> +    /*
> >>>> +     * If vIOMMU supports stage-1 translation, force to create nested
> >>> parent
> >>> I would rather not use another terminology here. You previously used
> >>> hw_nested, I think that's better. Also bear in mind that smmu supports
> >>> S1, S2 and S1+S2 in emulated code.
> >> What about 'nesting parent' to match kernel side terminology, per Nicolin's
> >suggestion:
> >>
> >> In kernel kdoc/uAPI, we use:
> >>  - "nesting parent" for stage-2 object
> >>  - "nested hwpt", "nested domain" for stage-1 object
> >I still think that since you queried the HW_NESTED cap it makes sense to
> >continue using it. This can come along with the kernel terminology though.
> 
> OK, like below, do I understand right?
> 
> +    /*
> +     * If vIOMMU supports stage-1 translation, force to create hw_nested
> +     * (aka. nesting parent in kernel) domain which could be reused by
> +     * vIOMMU to create nested domain.
> +     */

FWIW, while I was targeting the word "nested parent", I think Eric
was commenting on the word "stage-1 translation".

The vSMMU code supports "stage-1", "stage-2", and even "nested" as
its full emulation modes (no HW acceleration). So, any word like
"stage-1 translation" or "nested S1" can be confusing to the vSMMU
folks, as neither of them necessarily means "HW_NESTED" that stands
for "HW-accelerated nested stage-1".

Also, "HW_NESTED" != "nesting parent". They're two different things.
Thus, "force to create hw_nested" isn't accurate. Here, we want to
create a "nesting parent" HWPT. There is no other alternative name,
IMHO, given this is essentially a kernel-defined object.

Anyway, if we all agree on the VIOMMU_FLAG_WANT_NESTING_PARENT, it
is not necessary to have this comment (at least the first part) --
we could still note that the nesting parent HWPT will be reused by
vIOMMU to create nested HWPTs, if you'd like to.

Thanks
Nicolin