linux-kernel - Re: [PATCH v2 02/10] iommu: Introduce a new iommu_group_replace

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Message-ID: <Y/gLiNs42UyO58zJ@nvidia.com>
Date:   Thu, 23 Feb 2023 20:57:44 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Nicolin Chen <nicolinc@...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 02/10] iommu: Introduce a new
 iommu_group_replace_domain() API

On Wed, Feb 22, 2023 at 02:11:39AM +0000, Tian, Kevin wrote:

> > > There was an attempt [1] to fix error unwind in iommu_attach_group(), by
> > > temporarily set group->domain to NULL before calling set_domain().
> > >
> > > Jason, I wonder why this recovering cannot be done in
> > > __iommu_group_set_domain() directly, e.g.:
> > >
> > > 	ret = __iommu_group_for_each_dev(group, new_domain,
> > > 					  iommu_group_do_attach_device);
> > > 	if (ret) {
> > > 		__iommu_group_for_each_dev(group, group->domain,
> > > 					  iommu_group_do_attach_device);
> > > 		return ret;
> > > 	}
> > > 	group->domain = new_domain;
> > 
> > We talked about this already, some times this is not the correct
> > recovery case, eg if we are going to a blocking domain we need to drop
> > all references to the prior domain, not put them back.
> > 
> > Failures are WARN_ON events not error recovery.
> > 
> 
> OK, I remember that. Then here looks we also need temporarily
> set group->domain to NULL before calling set_domain() to recover,
> as [1] does.

Sigh, this is too much.

I made a series to clean up all the domain attach logic so the error
handling is all in one place and all the same.

What do you think?

https://github.com/jgunthorpe/linux/commits/iommufd_hwpt

Jason