linux-kernel - RE: [PATCH v2 5/5] vfio/iommu_type1: Simplify group attachment

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Date:   Fri, 17 Jun 2022 02:53:13 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Nicolin Chen <nicolinc@...dia.com>
CC:     "joro@...tes.org" <joro@...tes.org>,
        "will@...nel.org" <will@...nel.org>,
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Subject: RE: [PATCH v2 5/5] vfio/iommu_type1: Simplify group attachment

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Friday, June 17, 2022 6:41 AM
> 
> > ...
> > > -     if (resv_msi) {
> > > +     if (resv_msi && !domain->msi_cookie) {
> > >               ret = iommu_get_msi_cookie(domain->domain,
> > > resv_msi_base);
> > >               if (ret && ret != -ENODEV)
> > >                       goto out_detach;
> > > +             domain->msi_cookie = true;
> > >       }
> >
> > why not moving to alloc_attach_domain() then no need for the new
> > domain field? It's required only when a new domain is allocated.
> 
> When reusing an existing domain that doesn't have an msi_cookie,
> we can do iommu_get_msi_cookie() if resv_msi is found. So it is
> not limited to a new domain.

Looks msi_cookie requirement is per platform (currently only
for smmu. see arm_smmu_get_resv_regions()). If there is
no mixed case then above check is not required.

But let's hear whether Robin has a different thought here.

> 
> > ...
> > > -             if (list_empty(&domain->group_list)) {
> > > -                     if (list_is_singular(&iommu->domain_list)) {
> > > -                             if (list_empty(&iommu-
> > > >emulated_iommu_groups)) {
> > > -                                     WARN_ON(iommu->notifier.head);
> > > -
> > >       vfio_iommu_unmap_unpin_all(iommu);
> > > -                             } else {
> > > -
> > >       vfio_iommu_unmap_unpin_reaccount(iommu);
> > > -                             }
> > > -                     }
> > > -                     iommu_domain_free(domain->domain);
> > > -                     list_del(&domain->next);
> > > -                     kfree(domain);
> > > -                     vfio_iommu_aper_expand(iommu, &iova_copy);
> >
> > Previously the aperture is adjusted when a domain is freed...
> >
> > > -                     vfio_update_pgsize_bitmap(iommu);
> > > -             }
> > > -             /*
> > > -              * Removal of a group without dirty tracking may allow
> > > -              * the iommu scope to be promoted.
> > > -              */
> > > -             if (!group->pinned_page_dirty_scope) {
> > > -                     iommu->num_non_pinned_groups--;
> > > -                     if (iommu->dirty_page_tracking)
> > > -                             vfio_iommu_populate_bitmap_full(iommu);
> > > -             }
> > > +             vfio_iommu_detach_destroy_domain(domain, iommu,
> > > group);
> > >               kfree(group);
> > >               break;
> > >       }
> > >
> > > +     vfio_iommu_aper_expand(iommu, &iova_copy);
> >
> > but now it's done for every group detach. The aperture is decided
> > by domain geometry which is not affected by attached groups.
> 
> Yea, I've noticed this part. Actually Jason did this change for
> simplicity, and I think it'd be safe to do so?

Perhaps detach_destroy() can return a Boolean to indicate whether
a domain is destroyed.