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

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Date:   Thu, 9 Feb 2023 20:01:00 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.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()

On Thu, Feb 09, 2023 at 01:13:07PM -0800, Nicolin Chen wrote:
> On Thu, Feb 09, 2023 at 04:00:52AM +0000, Tian, Kevin wrote:
>  
> > > From: Nicolin Chen <nicolinc@...dia.com>
> > > Sent: Wednesday, February 8, 2023 5:18 AM
> > >
> > > iommu_group_replace_domain() is introduced to support use cases where
> > > an
> > > iommu_group can be attached to a new domain without getting detached
> > > from
> > > the old one. This replacement feature will be useful, for cases such as:
> > > 1) vPASID mode, when a guest wants to replace a single pasid (PASID=0)
> > >    table with a larger table (PASID=N)
> > > 2) Nesting mode, when switching the attaching device from an S2 domain
> > >    to an S1 domain, or when switching between relevant S1 domains.
> > > as it allows these cases to switch seamlessly without a DMA disruption.
> > >
> > > So, call iommu_group_replace_domain() in the iommufd_device_do_attach().
> > > And add a __iommmufd_device_detach helper to allow the replace routine
> > > to
> > > do a partial detach on the current hwpt that's being replaced. Though the
> > > updated locking logic is overcomplicated, it will be eased, once those
> > > iopt_table_add/remove_ioas and list_add/del calls are moved to hwpt's
> > > allocation/destroy() functions in the coming nesting series, as that'll
> > > depend on a new ->domain_alloc_user op in the iommu core.
> > 
> > then why not moving those changes into this series to make it simple?
> 
> The simplification depends on the new ->domain_alloc_user op and
> its implementation in SMMU driver, which would be introduced by
> the nesting series of VT-d and SMMU respectively.

Since we are not fixing all the drivers at this point, this argument
doesn't hold water.

It is as I said in the other email, there should be no changes to the
normal attach/replace path when adding manual HWPT creation - once
those are removed there should be minimal connection between these two
series.

Jason