linux-kernel - Re: [RFC PATCH 13/15] iommufd: Persist iommu domains for live update

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Message-ID: <20250930135916.GN2695987@ziepe.ca>
Date: Tue, 30 Sep 2025 10:59:16 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Samiullah Khawaja <skhawaja@...gle.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, iommu@...ts.linux.dev,
	YiFei Zhu <zhuyifei@...gle.com>,
	Robin Murphy <robin.murphy@....com>,
	Pratyush Yadav <pratyush@...nel.org>,
	Kevin Tian <kevin.tian@...el.com>, linux-kernel@...r.kernel.org,
	Saeed Mahameed <saeedm@...dia.com>,
	Adithya Jayachandran <ajayachandra@...dia.com>,
	Parav Pandit <parav@...dia.com>,
	Leon Romanovsky <leonro@...dia.com>, William Tu <witu@...dia.com>,
	Vipin Sharma <vipinsh@...gle.com>, dmatlack@...gle.com,
	Chris Li <chrisl@...nel.org>, praan@...gle.com
Subject: Re: [RFC PATCH 13/15] iommufd: Persist iommu domains for live update

On Tue, Sep 30, 2025 at 09:07:48AM -0400, Pasha Tatashin wrote:
> On Mon, Sep 29, 2025 at 12:00 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Sun, Sep 28, 2025 at 07:06:21PM +0000, Samiullah Khawaja wrote:
> > > +static int iommufd_save_ioas(struct iommufd_ctx *ictx,
> > > +                          struct iommufd_lu *iommufd_lu)
> > > +{
> > > +     struct iommufd_hwpt_paging *hwpt_paging;
> > > +     struct iommufd_ioas *ioas = NULL;
> > > +     struct iommufd_object *obj;
> > > +     unsigned long index;
> > > +     int rc;
> > > +
> > > +     /* Iterate each ioas. */
> > > +     xa_for_each(&ictx->objects, index, obj) {
> > > +             if (obj->type != IOMMUFD_OBJ_IOAS)
> > > +                     continue;
> >
> > Wrong locking
> >
> > > +
> > > +             ioas = (struct iommufd_ioas *)obj;
> > > +             mutex_lock(&ioas->mutex);
> > > +
> > > +             /*
> > > +              * TODO: Iterate over each device of this iommufd and only save
> > > +              * hwpt/domain if the device is persisted.
> > > +              */
> > > +             list_for_each_entry(hwpt_paging, &ioas->hwpt_list, hwpt_item) {
> > > +                     if (!hwpt_paging->common.domain)
> > > +                             continue;
> >
> > I don't think this should be automatic. The user should directly
> > serialize/unserialize HWPTs by ID.
> 
> Why not?  Live Updated uAPI is handled through FDs, and both iommufd
> and vfiofd have to be preserved; I assume we can automatically
> determine the hwpt to be preserved through dependencies. Why would we
> delegate this to the user?

There are HWPTs outside the IOAS so it is inconsisent.

We are not going to reconstruct the IOAS.

The IDR ids of the HWPT may not be available on restore (we cannot
make this ABI), so without userspace expressly labeling them and
recovering the new IDR ids it doesn't work.

Finally we expect to discard the preserved HWPTs and replace them we
rebuilt ones at least as a first step. Userspace needs to sequence all
of this..

Jason