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Date:   Fri, 10 Feb 2023 12:50:14 -0800
From:   Nicolin Chen <nicolinc@nvidia.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
CC:     "Tian, Kevin" <kevin.tian@intel.com>,
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Subject: Re: [PATCH v2 08/10] iommufd/device: Use iommu_group_replace_domain()
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On Thu, Feb 09, 2023 at 08:01:00PM -0400, Jason Gunthorpe wrote:
> 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@nvidia.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.

Yes. I replied here earlier than the discussion with you in
that thread. I think I should just drop this line of commit
messages.

Thanks
Nic