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From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Jason Gunthorpe <jgg@nvidia.com>,
        Alex Williamson <alex.williamson@redhat.com>
CC:     Jean-Philippe Brucker <jean-philippe@linaro.org>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Jean-Philippe Brucker <jean-philippe@linaro.com>,
        Li Zefan <lizefan@huawei.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tejun Heo <tj@kernel.org>,
        "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
        "Wu, Hao" <hao.wu@intel.com>, David Woodhouse <dwmw2@infradead.org>
Subject: RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation
 APIs
Thread-Topic: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs
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Date:   Sat, 8 May 2021 06:08:33 +0000
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> From: Jason Gunthorpe
> Sent: Saturday, May 8, 2021 1:11 AM
>=20
> On Fri, May 07, 2021 at 11:06:14AM -0600, Alex Williamson wrote:
>=20
> > We had tossed around an idea of a super-container with vfio, it's maybe
> > something we'd want to incorporate into this design.  For instance, if
> > memory could be pre-registered with a super container, which would
> > handle the locked memory accounting for that memory, then
> > sub-containers could all handle the IOMMU context of their sets of
> > devices relative to that common memory pool.
>=20
> This is where I suggested to David to use nesting of IOASIDs.
>=20
> Without HW support for nesting a SW nest is really just re-using the
> memory registration information stored in the parent when constructing
> the children
>=20

yes, this sounds a sensible thing to do. it also unifies the user experienc=
e
regardless of whether the underlying hw supports nesting, e.g. when=20
vIOMMU is present Qemu can always use IOASID nesting uAPI. In case
of SW nest then the kernel will merge the two-level translations from two
IOASIDs into one-level shadow page table (unlike today's VFIO which has=20
the userspace to manage shadow-based mapping).=20

but want to remark that nesting IOASIDs alone cannot solve this accounting
problem completely, as long as a process is allowed to have multiple ioasid
FDs (unless there is a mechanism to allow nesting IOASIDs cross FDs). But
this is probably not a big issue. With all the intended usages around the
new interface, I think for most applications one ioasid FD should be suffic=
ient
to meet their requirements (multiple gpa_ioasids, ioasid nesting, etc.).

Thanks
Kevin