Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable nested SMMUv3

[Peter Maydell <peter.maydell@linaro.org>]

On Fri, 13 Dec 2024 at 12:46, Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> On Fri, Dec 13, 2024 at 12:00:43PM +0000, Daniel P. Berrangé wrote:
> > On Fri, Nov 08, 2024 at 12:52:37PM +0000, Shameer Kolothum via wrote:
> > > Hi,
> > >
> > > This series adds initial support for a user-creatable "arm-smmuv3-nested"
> > > device to Qemu. At present the Qemu ARM SMMUv3 emulation is per machine
> > > and cannot support multiple SMMUv3s.
> > >
> > > In order to support vfio-pci dev assignment with vSMMUv3, the physical
> > > SMMUv3 has to be configured in nested mode. Having a pluggable
> > > "arm-smmuv3-nested" device enables us to have multiple vSMMUv3 for Guests
> > > running on a host with multiple physical SMMUv3s. A few benefits of doing
> > > this are,
> >
> > I'm not very familiar with arm, but from this description I'm not
> > really seeing how "nesting" is involved here. You're only talking
> > about the host and 1 L1 guest, no L2 guest.
>
> nesting is the term the iommu side is using to refer to the 2
> dimensional paging, ie a guest page table on top of a hypervisor page
> table.

Isn't that more usually called "two stage" paging? Calling
that "nesting" seems like it is going to be massively confusing...

Also, how does it relate to what this series seems to be
doing, where we provide the guest with two separate SMMUs?
(Are those two SMMUs "nested" in the sense that one is sitting
behind the other?)

thanks
-- PMM