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

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Message-ID: <20250929174310.GH2695987@ziepe.ca>
Date: Mon, 29 Sep 2025 14:43:10 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Samiullah Khawaja <skhawaja@...gle.com>
Cc: David Woodhouse <dwmw2@...radead.org>,
	Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>,
	Pasha Tatashin <pasha.tatashin@...een.com>, 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 Mon, Sep 29, 2025 at 10:32:22AM -0700, Samiullah Khawaja wrote:
> On Mon, Sep 29, 2025 at 9:00 AM 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.
> Interesting. So the user should be able to serialize/unserialize HWPTs
> before the Live Update PREPARE event? But what if a device was marked
> for preservation but the user never serialized the attached HWPT,
> would that be considered an error during LUO PREPARE or should iommufd
> serialize the remaining HWPTs here?

yes that would be an error

I also think your patch series is a bit upside down, you should
present the iommufd and core pieces first, then come with a driver
implementation last.

It will be easier to understand the context that having a driver
implementation appear out of no where with no callers..

And everything should be driven by iommufd in this step, the iommu
driver should not be magically auto-preserving itself. Just preserve
the drivers linked to devices being preserved by iommufd.

Jason