linux-kernel - RE: [PATCH v2 08/10] iommufd/device: Use iommu_group_replace

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Date:   Wed, 15 Feb 2023 01:38:32 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Nicolin Chen <nicolinc@...dia.com>
CC:     "jgg@...dia.com" <jgg@...dia.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>
Subject: RE: [PATCH v2 08/10] iommufd/device: Use iommu_group_replace_domain()

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Tuesday, February 14, 2023 7:00 PM
> 
> On Fri, Feb 10, 2023 at 02:11:23AM +0000, Tian, Kevin wrote:
> 
> > My confusion is that we have different flows between detach/attach
> > and replace.
> >
> > today with separate detach+attach we have following flow:
> >
> >         Remove device from current hwpt;
> >         if (last_device in hwpt) {
> >                 Remove hwpt domain from current iopt;
> >                 if (last_device in group)
> >                         detach group from hwpt domain;
> >         }
> >
> >         if (first device in group) {
> >                 attach group to new hwpt domain;
> >                 if (first_device in hwpt)
> >                         Add hwpt domain to new iopt;
> >         Add device to new hwpt;
> >
> > but replace flow is different on the detach part:
> >
> >         if (first device in group) {
> >                 replace group's domain from current hwpt to new hwpt;
> >                 if (first_device in hwpt)
> >                         Add hwpt domain to new iopt;
> >         }
> >
> >         Remove device from old hwpt;
> >         if (last_device in old hwpt)
> >                 Remove hwpt domain from old iopt;
> >
> >         Add device to new hwpt;
> >
> > I'm yet to figure out whether we have sufficient lock protection to
> > prevent other paths from using old iopt/hwpt to find the device
> > which is already attached to a different domain.
> 
> With Jason's new series, the detach() routine is lighter now.
> 
> I wonder if it'd be safer now to do the detach() call after
> iommu_group_replace_domain()?
> 

yes, looks so.